Important

You are viewing the documentation of TiDB v8.3 (DMR). PingCAP does not provide bug fixes for v8.3. Any bugs will be fixed in future releases. For general purposes, please use the latest LTS version of the TiDB database.

System Variables

TiDB system variables behave similar to MySQL, in that settings apply on a SESSION or GLOBAL scope:

Changes on a SESSION scope will only affect the current session.
Changes on a GLOBAL scope apply immediately. If this variable is alsoSESSION scoped, all sessions (including your session) will continue to use their current session value.
Changes are made using the SET statement:

# These two identical statements change a session variable
SET tidb_distsql_scan_concurrency = 10;
SET SESSION tidb_distsql_scan_concurrency = 10;

# These two identical statements change a global variable
SET @@global.tidb_distsql_scan_concurrency = 10;
SET GLOBAL tidb_distsql_scan_concurrency = 10;

Note

Several GLOBAL variables persist to the TiDB cluster. Some variables in this document have a Persists to cluster setting, which can be configured to Yes or No.

For variables with the Persists to cluster: Yes setting, when a global variable is changed, a notification is sent to all TiDB servers to refresh their system variable cache. When you add additional TiDB servers or restart existing TiDB servers, the persisted configuration value is automatically used.
For variables with the Persists to cluster: No setting, changes only apply to the local TiDB instance that you are connected to. To retain any values set, you need to specify the variables in your tidb.toml configuration file.

Additionally, TiDB presents several MySQL variables as both readable and settable. This is required for compatibility, because it is common for both applications and connectors to read MySQL variables. For example, JDBC connectors both read and set query cache settings, despite not relying on the behavior.

Note

Larger values do not always yield better performance. It is also important to consider the number of concurrent connections that are executing statements, because most settings apply to each connection.

Consider the unit of a variable when you determine safe values:

For threads, safe values are typically up to the number of CPU cores.
For bytes, safe values are typically less than the amount of system memory.
For time, pay attention that the unit might be seconds or milliseconds.

Variables using the same unit might compete for the same set of resources.

Starting from v7.4.0, you can temporarily modify the value of some SESSION variables during statement execution using SET_VAR. After the statement is executed, the value of the system variable in the current session is automatically changed back to the original value. This hint can be used to modify some system variables related to the optimizer and executor. Variables in this document have a Applies to hint SET_VAR setting, which can be configured to Yes or No.

For variables with the Applies to hint SET_VAR: Yes setting, you can use the SET_VAR hint to modify the value of the system variable in the current session during statement execution.
For variables with the Applies to hint SET_VAR: No setting, you cannot use the SET_VAR hint to modify the value of the system variable in the current session during statement execution.

For more information about the SET_VAR hint, see SET_VAR.

Variable reference

allow_auto_random_explicit_insert New in v4.0.3

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: OFF
Determines whether to allow explicitly specifying the values of the column with the AUTO_RANDOM attribute in the INSERT statement.

authentication_ldap_sasl_auth_method_name New in v7.1.0

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Enumeration
Default value: SCRAM-SHA-1
Possible values: SCRAM-SHA-1, SCRAM-SHA-256, and GSSAPI.
For LDAP SASL authentication, this variable specifies the authentication method name.

authentication_ldap_sasl_bind_base_dn New in v7.1.0

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: String
Default value: ""
For LDAP SASL authentication, this variable limits the search scope within the search tree. If a user is created without the AS ... clause, TiDB will automatically search the dn in LDAP server according to the user name.

authentication_ldap_sasl_bind_root_dn New in v7.1.0

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: String
Default value: ""
For LDAP SASL authentication, this variable specifies the dn used to log in to the LDAP server to search users.

authentication_ldap_sasl_bind_root_pwd New in v7.1.0

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: String
Default value: ""
For LDAP SASL authentication, this variable specifies the password used to log in to the LDAP server to search users.

authentication_ldap_sasl_ca_path New in v7.1.0

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: String
Default value: ""
For LDAP SASL authentication, this variable specifies the absolute path of the certificate authority file for StartTLS connections.

authentication_ldap_sasl_init_pool_size New in v7.1.0

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 10
Range: [1, 32767]
For LDAP SASL authentication, this variable specifies the initial connections in the connection pool to the LDAP server.

authentication_ldap_sasl_max_pool_size New in v7.1.0

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 1000
Range: [1, 32767]
For LDAP SASL authentication, this variable specifies the maximum connections in the connection pool to the LDAP server.

authentication_ldap_sasl_server_host New in v7.1.0

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: String
Default value: ""
For LDAP SASL authentication, this variable specifies the LDAP server host name or IP address.

authentication_ldap_sasl_server_port New in v7.1.0

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 389
Range: [1, 65535]
For LDAP SASL authentication, this variable specifies the TCP/IP port number of the LDAP server.

authentication_ldap_sasl_tls New in v7.1.0

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: OFF
For LDAP SASL authentication, this variable controls whether connections by the plugin to the LDAP server are protected with StartTLS.

authentication_ldap_simple_auth_method_name New in v7.1.0

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Enumeration
Default value: SIMPLE
Possible values: SIMPLE.
For LDAP simple authentication, this variable specifies the authentication method name. The only supported value is SIMPLE.

authentication_ldap_simple_bind_base_dn New in v7.1.0

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: String
Default value: ""
For LDAP simple authentication, this variable limits the search scope within the search tree. If a user is created without the AS ... clause, TiDB will automatically search the dn in LDAP server according to the user name.

authentication_ldap_simple_bind_root_dn New in v7.1.0

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: String
Default value: ""
For LDAP simple authentication, this variable specifies the dn used to log in to the LDAP server to search users.

authentication_ldap_simple_bind_root_pwd New in v7.1.0

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: String
Default value: ""
For LDAP simple authentication, this variable specifies the password used to log in to the LDAP server to search users.

authentication_ldap_simple_ca_path New in v7.1.0

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: String
Default value: ""
For LDAP simple authentication, this variable specifies the absolute path of the certificate authority file for StartTLS connections.

authentication_ldap_simple_init_pool_size New in v7.1.0

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 10
Range: [1, 32767]
For LDAP simple authentication, this variable specifies the initial connections in the connection pool to the LDAP server.

authentication_ldap_simple_max_pool_size New in v7.1.0

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 1000
Range: [1, 32767]
For LDAP simple authentication, this variable specifies the maximum connections in the connection pool to the LDAP server.

authentication_ldap_simple_server_host New in v7.1.0

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: String
Default value: ""
For LDAP simple authentication, this variable specifies the LDAP server host name or IP address.

authentication_ldap_simple_server_port New in v7.1.0

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 389
Range: [1, 65535]
For LDAP simple authentication, this variable specifies the TCP/IP port number of the LDAP server.

authentication_ldap_simple_tls New in v7.1.0

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: OFF
For LDAP simple authentication, this variable controls whether connections by the plugin to the LDAP server are protected with StartTLS.

auto_increment_increment

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 1
Range: [1, 65535]
Controls the step size of AUTO_INCREMENT values to be allocated to a column, and allocation rules for AUTO_RANDOM IDs. It is often used in combination with auto_increment_offset.

auto_increment_offset

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 1
Range: [1, 65535]
Controls the initial offset of AUTO_INCREMENT values to be allocated to a column, and allocation rules for AUTO_RANDOM IDs. This setting is often used in combination with auto_increment_increment. For example:

mysql> CREATE TABLE t1 (a int not null primary key auto_increment);
Query OK, 0 rows affected (0.10 sec)

mysql> set auto_increment_offset=1;
Query OK, 0 rows affected (0.00 sec)

mysql> set auto_increment_increment=3;
Query OK, 0 rows affected (0.00 sec)

mysql> INSERT INTO t1 VALUES (),(),(),();
Query OK, 4 rows affected (0.04 sec)
Records: 4  Duplicates: 0  Warnings: 0

mysql> SELECT * FROM t1;
+----+
| a  |
+----+
|  1 |
|  4 |
|  7 |
| 10 |
+----+
4 rows in set (0.00 sec)

autocommit

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: ON
Controls whether statements should automatically commit when not in an explicit transaction. See Transaction Overview for more information.

block_encryption_mode

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Enumeration
Default value: aes-128-ecb
Value options: aes-128-ecb, aes-192-ecb, aes-256-ecb, aes-128-cbc, aes-192-cbc, aes-256-cbc, aes-128-ofb, aes-192-ofb, aes-256-ofb, aes-128-cfb, aes-192-cfb, aes-256-cfb
This variable sets the encryption mode for the built-in functions AES_ENCRYPT() and AES_DECRYPT().

character_set_client

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Default value: utf8mb4
The character set for data sent from the client. See Character Set and Collation for details on the use of character sets and collations in TiDB. It is recommended to use SET NAMES to change the character set when needed.

character_set_connection

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Default value: utf8mb4
The character set for string literals that do not have a specified character set.

character_set_database

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Default value: utf8mb4
This variable indicates the character set of the default database in use. It is NOT recommended to set this variable. When a new default database is selected, the server changes the variable value.

character_set_results

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Default value: utf8mb4
The character set that is used when data is sent to the client.

character_set_server

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Default value: utf8mb4
The default character set for the server.

collation_connection

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Default value: utf8mb4_bin
This variable indicates the collation used in the current connection. It is consistent with the MySQL variable collation_connection.

collation_database

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Default value: utf8mb4_bin
This variable indicates the default collation of the database in use. It is NOT recommended to set this variable. When a new database is selected, TiDB changes this variable value.

collation_server

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Default value: utf8mb4_bin
The default collation used when the database is created.

cte_max_recursion_depth

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Integer
Default value: 1000
Range: [0, 4294967295]
Controls the maximum recursion depth in Common Table Expressions.

datadir

Note

This variable is not supported on TiDB Serverless.

Scope: NONE
Applies to hint SET_VAR: No
Default value: it depends on the component and the deployment method.
- /tmp/tidb: when you set "unistore" for --store or if you don't set --store.
- ${pd-ip}:${pd-port}: when you use TiKV, which is the default storage engine for TiUP and TiDB Operator for Kubernetes deployments.
This variable indicates the location where data is stored. This location can be a local path /tmp/tidb, or point to a PD server if the data is stored on TiKV. A value in the format of ${pd-ip}:${pd-port} indicates the PD server that TiDB connects to on startup.

ddl_slow_threshold

Scope: GLOBAL
Persists to cluster: No, only applicable to the current TiDB instance that you are connecting to.
Applies to hint SET_VAR: No
Type: Integer
Default value: 300
Range: [0, 2147483647]
Unit: Milliseconds
Log DDL operations whose execution time exceeds the threshold value.

default_authentication_plugin

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Enumeration
Default value: mysql_native_password
Possible values: mysql_native_password, caching_sha2_password, tidb_sm3_password, tidb_auth_token, authentication_ldap_sasl, and authentication_ldap_simple.
This variable sets the authentication method that the server advertises when the server-client connection is being established.
To authenticate using the tidb_sm3_password method, you can connect to TiDB using TiDB-JDBC.

For more possible values of this variable, see Authentication plugin status.

default_collation_for_utf8mb4 New in v7.4.0

Scope: GLOBAL | SESSION
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: String
Default value: utf8mb4_bin
Value options: utf8mb4_bin, utf8mb4_general_ci, utf8mb4_0900_ai_ci
This variable is used to set the default collation for the utf8mb4 character set. It affects the behavior of the following statements:
- The default collation displayed in the SHOW COLLATION and SHOW CHARACTER SET statements.
- If CREATE TABLE and ALTER TABLE statements contain the CHARACTER SET utf8mb4 clause against a table or a column without specifying a collation, the collation specified by this variable is used. This does not affect the behavior when CHARACTER SET clause is not used.
- If CREATE DATABASE and ALTER DATABASE statements contain CHARACTER SET utf8mb4 clause without specifying a collation, the collation specified by this variable is used. This does not affect the behavior when CHARACTER SET clause is not used.
- If the COLLATE clause is not used, any literal string in the format of _utf8mb4'string' uses the collation specified by this variable.

default_password_lifetime New in v6.5.0

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 0
Range: [0, 65535]
Sets the global policy for automatic password expiration. The default value 0 indicates that the password never expires. If this system variable is set to a positive integer N, it means that the password lifetime is N days, and you must change your password within N days.

default_week_format

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 0
Range: [0, 7]
Sets the week format used by the WEEK() function.

disconnect_on_expired_password New in v6.5.0

Scope: GLOBAL
Applies to hint SET_VAR: No
Type: Boolean
Default value: ON
This variable is read-only. It indicates whether TiDB disconnects the client connection when the password is expired. If the variable is set to ON, the client connection is disconnected when the password is expired. If the variable is set to OFF, the client connection is restricted to the "sandbox mode" and the user can only execute the password reset operation.

If you need to change the behavior of the client connection for the expired password, modify the security.disconnect-on-expired-password configuration item in the configuration file.

div_precision_increment New in v8.0.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Integer
Default value: 4
Range: [0, 30]
This variable specifies the number of digits by which to increase the scale of the result of a division operation performed using the / operator. This variable is the same as MySQL.

error_count

Scope: SESSION
Applies to hint SET_VAR: No
Type: Integer
Default value: 0
A read-only variable that indicates the number of errors that resulted from the last statement that generated messages.

foreign_key_checks

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: Before v6.6.0, the default value is OFF. Starting from v6.6.0, the default value is ON.
This variable controls whether to enable foreign key constraint checking.

group_concat_max_len

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 1024
Range: [4, 18446744073709551615]
The maximum buffer size for items in the GROUP_CONCAT() function.

have_openssl

Scope: NONE
Applies to hint SET_VAR: No
Type: Boolean
Default value: DISABLED
A read-only variable for MySQL compatibility. Set to YES by the server when the server has TLS enabled.

have_ssl

Scope: NONE
Applies to hint SET_VAR: No
Type: Boolean
Default value: DISABLED
A read-only variable for MySQL compatibility. Set to YES by the server when the server has TLS enabled.

hostname

Scope: NONE
Applies to hint SET_VAR: No
Default value: (system hostname)
The hostname of the TiDB server as a read-only variable.

identity New in v5.3.0

This variable is an alias for last_insert_id.

init_connect

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Default value: ""
The init_connect feature permits a SQL statement to be automatically executed when you first connect to a TiDB server. If you have the CONNECTION_ADMIN or SUPER privileges, this init_connect statement will not be executed. If the init_connect statement results in an error, your user connection will be terminated.

innodb_lock_wait_timeout

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 50
Range: [1, 3600]
Unit: Seconds
The lock wait timeout for pessimistic transactions (default).

interactive_timeout

Note

This variable is read-only for TiDB Serverless.

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 28800
Range: [1, 31536000]
Unit: Seconds
This variable represents the idle timeout of the interactive user session. Interactive user session refers to the session established by calling mysql_real_connect() API using the CLIENT_INTERACTIVE option (for example, MySQL Shell and MySQL Client). This variable is fully compatible with MySQL.

last_insert_id

Scope: SESSION
Applies to hint SET_VAR: No
Type: Integer
Default value: 0
Range: [0, 18446744073709551615]
This variable returns the last AUTO_INCREMENT or AUTO_RANDOM value generated by an insert statement.
The value of last_insert_id is the same as the value returned by the function LAST_INSERT_ID().

last_plan_from_binding New in v4.0

Scope: SESSION
Applies to hint SET_VAR: No
Type: Boolean
Default value: OFF
This variable is used to show whether the execution plan used in the previous statement was influenced by a plan binding

last_plan_from_cache New in v4.0

Scope: SESSION
Applies to hint SET_VAR: No
Type: Boolean
Default value: OFF
This variable is used to show whether the execution plan used in the previous execute statement is taken directly from the plan cache.

last_sql_use_alloc New in v6.4.0

Scope: SESSION
Applies to hint SET_VAR: No
Default value: OFF
This variable is read-only. It is used to show whether the previous statement uses a cached chunk object (chunk allocation).

license

Scope: NONE
Applies to hint SET_VAR: No
Default value: Apache License 2.0
This variable indicates the license of your TiDB server installation.

log_bin

Scope: NONE
Applies to hint SET_VAR: No
Type: Boolean
Default value: OFF
This variable indicates whether TiDB Binlog (deprecated) is used.

max_allowed_packet New in v6.1.0

Note

This variable is read-only for TiDB Serverless.

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Default value: 67108864
Range: [1024, 1073741824]
The value should be an integer multiple of 1024. If the value is not divisible by 1024, a warning will be prompted and the value will be rounded down. For example, when the value is set to 1025, the actual value in TiDB is 1024.
The maximum packet size allowed by the server and the client in one transmission of packets.
In the SESSION scope, this variable is read-only.
This variable is compatible with MySQL.

password_history New in v6.5.0

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 0
Range: [0, 4294967295]
This variable is used to establish a password reuse policy that allows TiDB to limit password reuse based on the number of password changes. The default value 0 means disabling the password reuse policy based on the number of password changes. When this variable is set to a positive integer N, the reuse of the last N passwords is not allowed.

mpp_exchange_compression_mode New in v6.6.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Default value: UNSPECIFIED
Value options: NONE, FAST, HIGH_COMPRESSION, UNSPECIFIED
This variable is used to specify the data compression mode of the MPP Exchange operator. This variable takes effect when TiDB selects the MPP execution plan with the version number 1. The meanings of the variable values are as follows:
- UNSPECIFIED: means unspecified. TiDB will automatically select the compression mode. Currently, TiDB automatically selects the FAST mode.
- NONE: no data compression is used.
- FAST: fast mode. The overall performance is good and the compression ratio is less than HIGH_COMPRESSION.
- HIGH_COMPRESSION: the high compression ratio mode.

mpp_version New in v6.6.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Default value: UNSPECIFIED
Value options: UNSPECIFIED, 0, 1, 2
This variable is used to specify different versions of the MPP execution plan. After a version is specified, TiDB selects the specified version of the MPP execution plan. The meanings of the variable values are as follows:
- UNSPECIFIED: means unspecified. TiDB automatically selects the latest version 2.
- 0: compatible with all TiDB cluster versions. Features with the MPP version greater than 0 do not take effect in this mode.
- 1: new in v6.6.0, used to enable data exchange with compression on TiFlash. For details, see MPP version and exchange data compression.
- 2: new in v7.3.0, used to provide more accurate error messages when MPP tasks encounter errors on TiFlash.

password_reuse_interval New in v6.5.0

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 0
Range: [0, 4294967295]
This variable is used to establish a password reuse policy that allows TiDB to limit password reuse based on time elapsed. The default value 0 means disabling the password reuse policy based on time elapsed. When this variable is set to a positive integer N, the reuse of any password used in the last N days is not allowed.

max_connections

Scope: GLOBAL
Persists to cluster: No, only applicable to the current TiDB instance that you are connecting to.
Applies to hint SET_VAR: No
Type: Integer
Default value: 0
Range: [0, 100000]
The maximum number of concurrent connections permitted for a single TiDB instance. This variable can be used for resources control.
The default value 0 means no limit. When the value of this variable is larger than 0, and the number of connections reaches the value, the TiDB server rejects new connections from clients.

max_execution_time

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Integer
Default value: 0
Range: [0, 2147483647]
Unit: Milliseconds
The maximum execution time of a statement. The default value is unlimited (zero).

Note

Before v6.4.0, the max_execution_time system variable takes effect on all types of statements. Starting from v6.4.0, this variable only controls the maximum execution time of read-only statements. The precision of the timeout value is roughly 100ms. This means the statement might not be terminated in exact milliseconds as you specify.

For a SQL statement with the MAX_EXECUTION_TIME hint, the maximum execution time of this statement is limited by the hint instead of this variable. The hint can also be used with SQL bindings as described in the SQL FAQ.

max_prepared_stmt_count

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: -1
Range: [-1, 1048576]
Specifies the maximum number of PREPARE statements in the current TiDB instance.
The value of -1 means no limit on the maximum number of PREPARE statements in the current TiDB instance.
If you set the variable to a value that exceeds the upper limit 1048576, 1048576 is used instead:

mysql> SET GLOBAL max_prepared_stmt_count = 1048577;
Query OK, 0 rows affected, 1 warning (0.01 sec)

mysql> SHOW WARNINGS;
+---------+------+--------------------------------------------------------------+
| Level   | Code | Message                                                      |
+---------+------+--------------------------------------------------------------+
| Warning | 1292 | Truncated incorrect max_prepared_stmt_count value: '1048577' |
+---------+------+--------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> SHOW GLOBAL VARIABLES LIKE 'max_prepared_stmt_count';
+-------------------------+---------+
| Variable_name           | Value   |
+-------------------------+---------+
| max_prepared_stmt_count | 1048576 |
+-------------------------+---------+
1 row in set (0.00 sec)

pd_enable_follower_handle_region New in v7.6.0

Warning

The Active PD Follower feature is experimental. It is not recommended that you use it in the production environment. This feature might be changed or removed without prior notice. If you find a bug, you can report an issue on GitHub.

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: OFF
This variable controls whether to enable the Active PD Follower feature (currently only applicable to requests for Region information). When the value is OFF, TiDB only obtains Region information from the PD leader. When the value is ON, TiDB evenly distributes requests for Region information to all PD servers, and PD followers can also handle Region requests, thereby reducing the CPU pressure on the PD leader.
Scenarios for enabling Active PD Follower:
- In a cluster with a large number of Regions, the PD leader experiences high CPU pressure due to the increased overhead of handling heartbeats and scheduling tasks.
- In a TiDB cluster with many TiDB instances, the PD leader experiences high CPU pressure due to a high concurrency of requests for Region information.

plugin_dir

Note

This variable is not supported on TiDB Serverless.

Scope: GLOBAL
Persists to cluster: No, only applicable to the current TiDB instance that you are connecting to.
Applies to hint SET_VAR: No
Default value: ""
Indicates the directory to load plugins as specified by a command-line flag.

plugin_load

Note

This variable is not supported on TiDB Serverless.

Scope: GLOBAL
Persists to cluster: No, only applicable to the current TiDB instance that you are connecting to.
Applies to hint SET_VAR: No
Default value: ""
Indicates the plugins to load when TiDB is started. These plugins are specified by a command-line flag and separated by commas.

port

Scope: NONE
Applies to hint SET_VAR: No
Type: Integer
Default value: 4000
Range: [0, 65535]
The port that the tidb-server is listening on when speaking the MySQL protocol.

rand_seed1

Scope: SESSION
Applies to hint SET_VAR: No
Type: Integer
Default value: 0
Range: [0, 2147483647]
This variable is used to seed the random value generator used in the RAND() SQL function.
The behavior of this variable is MySQL compatible.

rand_seed2

Scope: SESSION
Applies to hint SET_VAR: No
Type: Integer
Default value: 0
Range: [0, 2147483647]
This variable is used to seed the random value generator used in the RAND() SQL function.
The behavior of this variable is MySQL compatible.

require_secure_transport New in v6.1.0

Note

Currently, this variable is not supported on TiDB Dedicated. DO NOT enable this variable for TiDB Dedicated clusters. Otherwise, you might get SQL client connection failures. This restriction is a temporary control measure and will be resolved in a future release.

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: OFF for TiDB Self-Hosted and TiDB Dedicated, ON for TiDB Serverless

This variable ensures that all connections to TiDB are either on a local socket, or using TLS. See Enable TLS between TiDB Clients and Servers for additional details.

Setting this variable to ON requires you to connect to TiDB from a session that has TLS enabled. This helps prevent lock-out scenarios when TLS is not configured correctly.
This setting was previously a tidb.toml option (security.require-secure-transport), but changed to a system variable starting from TiDB v6.1.0.
Starting from v6.5.6, v7.1.2, v7.5.1, and v8.0.0, when Security Enhanced Mode (SEM) is enabled, setting this variable to ON is prohibited to avoid potential connectivity issues for users.

skip_name_resolve New in v5.2.0

Note

This variable is read-only for TiDB Serverless.

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: OFF
This variable controls whether the tidb-server instance resolves hostnames as a part of the connection handshake.
When the DNS is unreliable, you can enable this option to improve network performance.

Note

When skip_name_resolve=ON, users with a hostname in their identity will no longer be able to log into the server. For example:

CREATE USER 'appuser'@'apphost' IDENTIFIED BY 'app-password';

In this example, it is recommended to replace apphost with an IP address or the wildcard (%).

socket

Scope: NONE
Applies to hint SET_VAR: No
Default value: ""
The local unix socket file that the tidb-server is listening on when speaking the MySQL protocol.

sql_log_bin

Note

This variable is read-only for TiDB Serverless.

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: ON
Indicates whether to write changes to TiDB Binlog or not.

Note

It is not recommended to set sql_log_bin as a global variable because the future versions of TiDB might only allow setting this as a session variable.

sql_mode

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Default value: ONLY_FULL_GROUP_BY,STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION
This variable controls a number of MySQL compatibility behaviors. See SQL Mode for more information.

sql_require_primary_key New in v6.3.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: OFF
This variable controls whether to enforce the requirement that a table has a primary key. After this variable is enabled, attempting to create or alter a table without a primary key will produce an error.
This feature is based on the similarly named sql_require_primary_key in MySQL 8.0.
It is strongly recommended to enable this variable when using TiCDC. This is because replicating changes to a MySQL sink requires that tables have a primary key.

If you enable this variable and are using TiDB Data Migration (DM) to migrate data, it is recommended that you add sql_require_ primary_key to the session part in the DM Task Configuration File and set it to OFF. Otherwise, it will cause DM to fail to create tasks.

sql_select_limit New in v4.0.2

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 18446744073709551615
Range: [0, 18446744073709551615]
Unit: Rows
The maximum number of rows returned by the SELECT statements.

ssl_ca

Scope: NONE
Applies to hint SET_VAR: No
Default value: ""
The location of the certificate authority file (if there is one). The value of this variable is defined by the TiDB configuration item ssl-ca.

ssl_cert

Scope: NONE
Applies to hint SET_VAR: No
Default value: ""
The location of the certificate file (if there is a file) that is used for SSL/TLS connections. The value of this variable is defined by the TiDB configuration item ssl-cert.

ssl_key

Scope: NONE
Applies to hint SET_VAR: No
Default value: ""
The location of the private key file (if there is one) that is used for SSL/TLS connections. The value of this variable is defined by TiDB configuration item ssl-key.

system_time_zone

Scope: NONE
Applies to hint SET_VAR: No
Default value: (system dependent)
This variable shows the system time zone from when TiDB was first bootstrapped. See also time_zone.

tidb_adaptive_closest_read_threshold New in v6.3.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 4096
Range: [0, 9223372036854775807]
Unit: Bytes
This variable is used to control the threshold at which the TiDB server prefers to send read requests to a replica in the same availability zone as the TiDB server when tidb_replica_read is set to closest-adaptive. If the estimated result is higher than or equal to this threshold, TiDB prefers to send read requests to a replica in the same availability zone. Otherwise, TiDB sends read requests to the leader replica.

tidb_allow_tiflash_cop New in v7.3.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: OFF
When TiDB pushes computation tasks down to TiFlash, there are three methods (or protocols) to choose from: Cop, BatchCop, and MPP. Compared to Cop and BatchCop, the MPP protocol is more mature and offers better task and resource management. Therefore, it is recommended to use the MPP protocol.
- 0 or OFF: the optimizer only generates plans using the TiFlash MPP protocol.
- 1 or ON: the optimizer determines whether to use the Cop, BatchCop, or MPP protocol to generate execution plans based on the cost estimation.

tidb_allow_batch_cop New in v4.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Integer
Default value: 1
Range: [0, 2]
This variable is used to control how TiDB sends a coprocessor request to TiFlash. It has the following values:
- 0: Never send requests in batches
- 1: Aggregation and join requests are sent in batches
- 2: All coprocessor requests are sent in batches

tidb_allow_fallback_to_tikv New in v5.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Default value: ""
This variable is used to specify a list of storage engines that might fall back to TiKV. If the execution of a SQL statement fails due to a failure of the specified storage engine in the list, TiDB retries executing this SQL statement with TiKV. This variable can be set to "" or "tiflash". When this variable is set to "tiflash", if TiFlash returns a timeout error (error code: ErrTiFlashServerTimeout), TiDB retries executing this SQL statement with TiKV.

tidb_allow_function_for_expression_index New in v5.2.0

Scope: NONE
Applies to hint SET_VAR: No
Default value: json_array, json_array_append, json_array_insert, json_contains, json_contains_path, json_depth, json_extract, json_insert, json_keys, json_length, json_merge_patch, json_merge_preserve, json_object, json_pretty, json_quote, json_remove, json_replace, json_search, json_set, json_storage_size, json_type, json_unquote, json_valid, lower, md5, reverse, tidb_shard, upper, vitess_hash
This read-only variable is used to show the functions that are allowed to be used for creating expression indexes.

tidb_allow_mpp New in v5.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Boolean
Default value: ON
Controls whether to use the MPP mode of TiFlash to execute queries. The value options are as follows:
- 0 or OFF, which means that the MPP mode will not be used. For v7.3.0 or a later version, if you set the value of this variable to 0 or OFF, you also need to enable the tidb_allow_tiflash_cop variable. Otherwise, queries might return errors.
- 1 or ON, which means that the optimizer determines whether to use the MPP mode based on the cost estimation (by default).

MPP is a distributed computing framework provided by the TiFlash engine, which allows data exchange between nodes and provides high-performance, high-throughput SQL algorithms. For details about the selection of the MPP mode, refer to Control whether to select the MPP mode.

tidb_allow_remove_auto_inc New in v2.1.18 and v3.0.4

Scope: SESSION
Applies to hint SET_VAR: No
Type: Boolean
Default value: OFF
This variable is used to set whether the AUTO_INCREMENT property of a column is allowed to be removed by executing ALTER TABLE MODIFY or ALTER TABLE CHANGE statements. It is not allowed by default.

tidb_analyze_column_options New in v8.3.0

Note

This variable only works when tidb_analyze_version is set to 2.
If you upgrade your TiDB cluster from a version earlier than v8.3.0 to v8.3.0 or later, this variable is set to ALL by default to keep the original behavior.
Starting from v8.3.0, for a newly deployed TiDB cluster, this variable is set to PREDICATE by default.

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Enumeration
Default value: PREDICATE
Value options:ALL, PREDICATE
This variable controls the behavior of the ANALYZE TABLE statement. Setting it to PREDICATE means only collecting statistics for predicate columns; setting it to ALL means collecting statistics for all columns. In scenarios where OLAP queries are used, it is recommended to set it to ALL, otherwise collecting statistics can result in a significant drop in query performance.

tidb_analyze_distsql_scan_concurrency New in v7.6.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 4
Range: [0, 4294967295]. In versions earlier than v8.2.0, the minimum value is 1. When you set it to 0, it adaptively adjusts the concurrency based on the cluster size.
This variable is used to set the concurrency of the scan operation when executing the ANALYZE operation.

tidb_analyze_partition_concurrency

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Default value: 2. The default value is 1 for v7.4.0 and earlier versions.
This variable specifies the concurrency of reading and writing statistics for a partitioned table when TiDB analyzes the partitioned table.

tidb_analyze_version New in v5.1.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 2
Range: [1, 2]
Controls how TiDB collects statistics.
- For TiDB Self-Hosted, the default value of this variable changes from 1 to 2 starting from v5.3.0.
- For TiDB Cloud, the default value of this variable changes from 1 to 2 starting from v6.5.0.
- If your cluster is upgraded from an earlier version, the default value of tidb_analyze_version does not change after the upgrade.
For detailed introduction about this variable, see Introduction to Statistics.

tidb_analyze_skip_column_types New in v7.2.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Default value: "json,blob,mediumblob,longblob,mediumtext,longtext". Before v8.2.0, the default value is "json,blob,mediumblob,longblob".
Possible values: "json,blob,mediumblob,longblob,text,mediumtext,longtext"
This variable controls which types of columns are skipped for statistics collection when executing the ANALYZE command to collect statistics. The variable is only applicable for tidb_analyze_version = 2. Even if you specify a column using ANALYZE TABLE t COLUMNS c1, ... , cn, no statistics will be collected for the specified column if its type is in tidb_analyze_skip_column_types.

mysql> SHOW CREATE TABLE t;
+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table                                                                                                                                                                                                             |
+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| t     | CREATE TABLE `t` (
  `a` int(11) DEFAULT NULL,
  `b` varchar(10) DEFAULT NULL,
  `c` json DEFAULT NULL,
  `d` blob DEFAULT NULL,
  `e` longblob DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin |
+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> SELECT @@tidb_analyze_skip_column_types;
+----------------------------------+
| @@tidb_analyze_skip_column_types |
+----------------------------------+
| json,blob,mediumblob,longblob    |
+----------------------------------+
1 row in set (0.00 sec)

mysql> ANALYZE TABLE t;
Query OK, 0 rows affected, 1 warning (0.05 sec)

mysql> SELECT job_info FROM mysql.analyze_jobs ORDER BY end_time DESC LIMIT 1;
+---------------------------------------------------------------------+
| job_info                                                            |
+---------------------------------------------------------------------+
| analyze table columns a, b with 256 buckets, 500 topn, 1 samplerate |
+---------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> ANALYZE TABLE t COLUMNS a, c;
Query OK, 0 rows affected, 1 warning (0.04 sec)

mysql> SELECT job_info FROM mysql.analyze_jobs ORDER BY end_time DESC LIMIT 1;
+------------------------------------------------------------------+
| job_info                                                         |
+------------------------------------------------------------------+
| analyze table columns a with 256 buckets, 500 topn, 1 samplerate |
+------------------------------------------------------------------+
1 row in set (0.00 sec)

tidb_auto_analyze_end_time

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Time
Default value: 23:59 +0000
This variable is used to restrict the time window that the automatic update of statistics is permitted. For example, to only allow automatic statistics updates between 1 AM and 3 AM in UTC time, set tidb_auto_analyze_start_time='01:00 +0000' and tidb_auto_analyze_end_time='03:00 +0000'.

tidb_auto_analyze_partition_batch_size New in v6.4.0

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Default value: 128. Before v7.6.0, the default value is 1.
Range: [1, 1024]
This variable specifies the number of partitions that TiDB automatically analyzes when analyzing a partitioned table (which means automatically collecting statistics on a partitioned table).
If the value of this variable is smaller than the number of partitions, TiDB automatically analyzes all partitions of the partitioned table in multiple batches. If the value of this variable is greater than or equal to the number of partitions, TiDB analyzes all partitions of the partitioned table at the same time.
If the number of partitions of a partitioned table is far greater than this variable value and the auto-analyze takes a long time, you can increase the value of this variable to reduce the time consumption.

tidb_auto_analyze_ratio

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Float
Default value: 0.5
Range: (0, 1]. The range for v8.0.0 and earlier versions is [0, 18446744073709551615].
This variable is used to set the threshold when TiDB automatically executes ANALYZE TABLE in a background thread to update table statistics. For example, a value of 0.5 means that auto-analyze is triggered when greater than 50% of the rows in a table have been modified. Auto-analyze can be restricted to only execute during certain hours of the day by specifying tidb_auto_analyze_start_time and tidb_auto_analyze_end_time.

Note

This feature requires the system variable tidb_enable_auto_analyze set to ON.

tidb_auto_analyze_start_time

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Time
Default value: 00:00 +0000
This variable is used to restrict the time window that the automatic update of statistics is permitted. For example, to only allow automatic statistics updates between 1 AM and 3 AM in UTC time, set tidb_auto_analyze_start_time='01:00 +0000' and tidb_auto_analyze_end_time='03:00 +0000'.

tidb_auto_build_stats_concurrency New in v6.5.0

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 1
Range: [1, 256]
This variable is used to set the concurrency of executing the automatic update of statistics.

tidb_backoff_lock_fast

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 10
Range: [1, 2147483647]
This variable is used to set the backoff time when the read request meets a lock.

tidb_backoff_weight

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 2
Range: [0, 2147483647]
This variable is used to increase the weight of the maximum time of TiDB backoff, that is, the maximum retry time for sending a retry request when an internal network or other component (TiKV, PD) failure is encountered. This variable can be used to adjust the maximum retry time and the minimum value is 1.
For example, the base timeout for TiDB to take TSO from PD is 15 seconds. When tidb_backoff_weight = 2, the maximum timeout for taking TSO is: base time * 2 = 30 seconds.
In the case of a poor network environment, appropriately increasing the value of this variable can effectively alleviate error reporting to the application end caused by timeout. If the application end wants to receive the error information more quickly, minimize the value of this variable.

tidb_batch_commit

Warning

It is NOT recommended to enable this variable.

Scope: SESSION
Applies to hint SET_VAR: No
Type: Boolean
Default value: OFF
The variable is used to control whether to enable the deprecated batch-commit feature. When this variable is enabled, a transaction might be split into multiple transactions by grouping a few statements and committed non-atomically, which is not recommended.

tidb_batch_delete

Warning

This variable is associated with the deprecated batch-dml feature, which might cause data corruption. Therefore, it is not recommended to enable this variable for batch-dml. Instead, use non-transactional DML.

Scope: SESSION
Applies to hint SET_VAR: No
Type: Boolean
Default value: OFF
This variable is used to control whether to enable the batch-delete feature, which is a part of the deprecated batch-dml feature. When this variable is enabled, DELETE statements might be split into multiple transactions and committed non-atomically. To make it work, you also need to enable tidb_enable_batch_dml and set a positive value for tidb_dml_batch_size, which is not recommended.

tidb_batch_insert

Warning

Scope: SESSION
Applies to hint SET_VAR: No
Type: Boolean
Default value: OFF
This variable is used to control whether to enable the batch-insert feature, which is a part of the deprecated batch-dml feature. When this variable is enabled, INSERT statements might be split into multiple transactions and committed non-atomically. To make it work, you also need to enable tidb_enable_batch_dml and set a positive value for tidb_dml_batch_size, which is not recommended.

tidb_batch_pending_tiflash_count New in v6.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 4000
Range: [0, 4294967295]
Specifies the maximum number of permitted unavailable tables when you use ALTER DATABASE SET TIFLASH REPLICA to add TiFlash replicas. If the number of unavailable tables exceeds this limit, the operation will be stopped or setting TiFlash replicas for the remaining tables will be very slow.

tidb_broadcast_join_threshold_count New in v5.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Integer
Default value: 10240
Range: [0, 9223372036854775807]
Unit: Rows
If the objects of the join operation belong to a subquery, the optimizer cannot estimate the size of the subquery result set. In this situation, the size is determined by the number of rows in the result set. If the estimated number of rows in the subquery is less than the value of this variable, the Broadcast Hash Join algorithm is used. Otherwise, the Shuffled Hash Join algorithm is used.
This variable will not take effect after tidb_prefer_broadcast_join_by_exchange_data_size is enabled.

tidb_broadcast_join_threshold_size New in v5.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Integer
Default value: 104857600 (100 MiB)
Range: [0, 9223372036854775807]
Unit: Bytes
If the table size is less than the value of the variable, the Broadcast Hash Join algorithm is used. Otherwise, the Shuffled Hash Join algorithm is used.
This variable will not take effect after tidb_prefer_broadcast_join_by_exchange_data_size is enabled.

tidb_build_stats_concurrency

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 2. The default value is 4 for v7.4.0 and earlier versions.
Range: [1, 256]
Unit: Threads
This variable is used to set the concurrency of executing the ANALYZE statement.
When the variable is set to a larger value, the execution performance of other queries is affected.

tidb_build_sampling_stats_concurrency New in v7.5.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Unit: Threads
Default value: 2
Range: [1, 256]
This variable is used to set the sampling concurrency in the ANALYZE process.
When the variable is set to a larger value, the execution performance of other queries is affected.

tidb_capture_plan_baselines New in v4.0

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: OFF
This variable is used to control whether to enable the baseline capturing feature. This feature depends on the statement summary, so you need to enable the statement summary before you use baseline capturing.
After this feature is enabled, the historical SQL statements in the statement summary are traversed periodically, and bindings are automatically created for SQL statements that appear at least twice.

tidb_cdc_write_source New in v6.5.0

Note

This variable is read-only for TiDB Serverless.

Scope: SESSION
Persists to cluster: No
Applies to hint SET_VAR: No
Type: Integer
Default value: 0
Range: [0, 15]
When this variable is set to a value other than 0, data written in this session is considered to be written by TiCDC. This variable can only be modified by TiCDC. Do not manually modify this variable in any case.

tidb_check_mb4_value_in_utf8

Note

This TiDB variable is not applicable to TiDB Cloud.

Scope: GLOBAL
Persists to cluster: No, only applicable to the current TiDB instance that you are connecting to.
Applies to hint SET_VAR: No
Type: Boolean
Default value: ON
This variable is used to enforce that the utf8 character set only stores values from the Basic Multilingual Plane (BMP). To store characters outside the BMP, it is recommended to use the utf8mb4 character set.
You might need to disable this option when upgrading your cluster from an earlier version of TiDB where the utf8 checking was more relaxed. For details, see FAQs After Upgrade.

tidb_checksum_table_concurrency

Scope: SESSION
Applies to hint SET_VAR: No
Type: Integer
Default value: 4
Range: [1, 256]
Unit: Threads
This variable is used to set the scan index concurrency of executing the ADMIN CHECKSUM TABLE statement.
When the variable is set to a larger value, the execution performance of other queries is affected.

tidb_committer_concurrency New in v6.1.0

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 128
Range: [1, 10000]
The number of goroutines for requests related to executing commit in the commit phase of the single transaction.
If the transaction to commit is too large, the waiting time for the flow control queue when the transaction is committed might be too long. In this situation, you can increase the configuration value to speed up the commit.
This setting was previously a tidb.toml option (performance.committer-concurrency), but changed to a system variable starting from TiDB v6.1.0.

tidb_config

Note

This TiDB variable is not applicable to TiDB Cloud.

Scope: GLOBAL
Persists to cluster: No, only applicable to the current TiDB instance that you are connecting to.
Applies to hint SET_VAR: No
Default value: ""
This variable is read-only. It is used to obtain the configuration information of the current TiDB server.

tidb_constraint_check_in_place

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: OFF
This variable only applies to optimistic transactions. For pessimistic transactions, use tidb_constraint_check_in_place_pessimistic instead.

When this variable is set to OFF, checking for duplicate values in unique indexes is deferred until the transaction commits. This helps improve performance but might be an unexpected behavior for some applications. See Constraints for details.

When setting tidb_constraint_check_in_place to OFF and using optimistic transactions:

tidb> create table t (i int key);
tidb> insert into t values (1);
tidb> begin optimistic;
tidb> insert into t values (1);
Query OK, 1 row affected
tidb> commit; -- Check only when a transaction is committed.
ERROR 1062 : Duplicate entry '1' for key 't.PRIMARY'

When setting tidb_constraint_check_in_place to ON and using optimistic transactions:

tidb> set @@tidb_constraint_check_in_place=ON;
tidb> begin optimistic;
tidb> insert into t values (1);
ERROR 1062 : Duplicate entry '1' for key 't.PRIMARY'

tidb_constraint_check_in_place_pessimistic New in v6.3.0

Scope: SESSION
Applies to hint SET_VAR: No
Type: Boolean

Default value: By default, the pessimistic-txn.constraint-check-in-place-pessimistic configuration item is true so the default value of this variable is ON. When pessimistic-txn.constraint-check-in-place-pessimistic is set to false, the default value of this variable is OFF.

This variable only applies to pessimistic transactions. For optimistic transactions, use tidb_constraint_check_in_place instead.
When this variable is set to OFF, TiDB defers the unique constraint check of a unique index (to the next time when executing a statement that requires a lock to the index or to the time when committing the transaction). This helps improve performance but might be an unexpected behavior for some applications. See Constraints for details.
Disabling this variable might cause TiDB to return a LazyUniquenessCheckFailure error in pessimistic transactions. When this error occurs, TiDB rolls back the current transaction.
When this variable is disabled, you cannot use SAVEPOINT in pessimistic transactions.

When this variable is disabled, committing a pessimistic transaction might return a Write conflict or Duplicate entry error. When such an error occurs, TiDB rolls back the current transaction.

When setting tidb_constraint_check_in_place_pessimistic to OFF and using pessimistic transactions:

set @@tidb_constraint_check_in_place_pessimistic=OFF;
create table t (i int key);
insert into t values (1);
begin pessimistic;
insert into t values (1);

Query OK, 1 row affected

tidb> commit; -- Check only when a transaction is committed.

ERROR 1062 : Duplicate entry '1' for key 't.PRIMARY'

When setting tidb_constraint_check_in_place_pessimistic to ON and using pessimistic transactions:

set @@tidb_constraint_check_in_place_pessimistic=ON;
begin pessimistic;
insert into t values (1);

ERROR 1062 : Duplicate entry '1' for key 't.PRIMARY'

tidb_cost_model_version New in v6.2.0

Note

Since TiDB v6.5.0, the newly created cluster uses Cost Model Version 2 by default. If you upgrade from a TiDB version earlier than v6.5.0 to v6.5.0 or later, the tidb_cost_model_version value does not change.
Switching the version of the cost model might cause changes to query plans.

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Integer
Default value: 2
Value options:
- 1: enables the Cost Model Version 1, which is used by default in TiDB v6.4.0 and earlier versions.
- 2: enables the Cost Model Version 2, which is generally available in TiDB v6.5.0 and is more accurate than the version 1 in internal tests.
The version of cost model affects the plan decision of optimizer. For more details, see Cost Model.

tidb_current_ts

Scope: SESSION
Applies to hint SET_VAR: No
Type: Integer
Default value: 0
Range: [0, 9223372036854775807]
This variable is read-only. It is used to obtain the timestamp of the current transaction.

tidb_ddl_disk_quota New in v6.3.0

Note

This variable is read-only for TiDB Serverless.

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 107374182400 (100 GiB)
Range: [107374182400, 1125899906842624] ([100 GiB, 1 PiB])
Unit: Bytes
This variable only takes effect when tidb_ddl_enable_fast_reorg is enabled. It sets the usage limit of local storage during backfilling when creating an index.

tidb_ddl_enable_fast_reorg New in v6.3.0

Note

If you are using a TiDB Dedicated cluster, to improve the speed for index creation using this variable, make sure that your TiDB cluster is hosted on AWS and your TiDB node size is at least 8 vCPU.
For TiDB Serverless clusters, this variable is read-only.

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: ON
This variable controls whether to enable the acceleration of ADD INDEX and CREATE INDEX to improve the speed of backfilling for index creation. Setting this variable value to ON can bring performance improvement for index creation on tables with a large amount of data.
Starting from v7.1.0, the index acceleration operation supports checkpoints. Even if the TiDB owner node is restarted or changed due to failures, TiDB can still recover progress from checkpoints that are automatically updated on a regular basis.
To verify whether a completed ADD INDEX operation is accelerated, you can execute the ADMIN SHOW DDL JOBS statement to see whether ingest is displayed in the JOB_TYPE column.

Note

Index acceleration requires a temp-dir that is writable and has enough free space. If the temp-dir is unusable, TiDB falls back to non-accelerated index building. It is recommended to put the temp-dir on a SSD disk.
Before you upgrade TiDB to v6.5.0 or later, it is recommended that you check whether the temp-dir path of TiDB is correctly mounted to an SSD disk. Make sure that the operating system user that runs TiDB has the read and write permissions for this directory. Otherwise, The DDL operations might experience unpredictable issues. This path is a TiDB configuration item, which takes effect after TiDB is restarted. Therefore, setting this configuration item before upgrading can avoid another restart.

tidb_enable_dist_task New in v7.1.0

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Default value: ON
This variable is used to control whether to enable the TiDB Distributed eXecution Framework (DXF). After the framework is enabled, the DXF tasks such as DDL and import will be distributedly executed and completed by multiple TiDB nodes in the cluster.
Starting from TiDB v7.1.0, the DXF supports distributedly executing the ADD INDEX statement for partitioned tables.
Starting from TiDB v7.2.0, the DXF supports distributedly executing the IMPORT INTO statement for import jobs.
Starting from TiDB v8.1.0, this variable is enabled by default. If you want to upgrade a cluster with the DXF enabled to v8.1.0 or later, disable the DXF (by setting tidb_enable_dist_task to OFF) before the upgrade, which avoids ADD INDEX operations during the upgrade causing data index inconsistency. After the upgrade, you can manually enable the DXF.
This variable is renamed from tidb_ddl_distribute_reorg.

tidb_cloud_storage_uri New in v7.4.0

Note

Currently, the Global Sort process consumes a large amount of computing and memory resources of TiDB nodes. In scenarios such as adding indexes online while user business applications are running, it is recommended to add new TiDB nodes to the cluster, configure the tidb_service_scope variable for these nodes, and connect to these nodes to create tasks. In this way, the distributed framework schedules tasks to these nodes, isolating the workload from other TiDB nodes to reduce the impact of executing backend tasks such as ADD INDEX and IMPORT INTO on user business applications.

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Default value: ""
This variable is used to specify the Amazon S3 cloud storage URI to enable Global Sort. After enabling the TiDB Distributed eXecution Framework (DXF), you can use the Global Sort feature by configuring the URI and pointing it to an appropriate cloud storage path with the necessary permissions to access the storage. For more details, see Amazon S3 URI format.
The following statements can use the Global Sort feature.
- The ADD INDEX statement.
- The IMPORT INTO statement for import jobs.

tidb_ddl_error_count_limit

Note

This variable is read-only for TiDB Serverless.

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 512
Range: [0, 9223372036854775807]
This variable is used to set the number of retries when the DDL operation fails. When the number of retries exceeds the parameter value, the wrong DDL operation is canceled.

tidb_ddl_flashback_concurrency New in v6.3.0

Note

This variable is read-only for TiDB Serverless.

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 64
Range: [1, 256]
This variable controls the concurrency of FLASHBACK CLUSTER.

tidb_ddl_reorg_batch_size

Note

This variable is read-only for TiDB Serverless.

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 256
Range: [32, 10240]
Unit: Rows
This variable is used to set the batch size during the re-organize phase of the DDL operation. For example, when TiDB executes the ADD INDEX operation, the index data needs to backfilled by tidb_ddl_reorg_worker_cnt (the number) concurrent workers. Each worker backfills the index data in batches.
- If tidb_ddl_enable_fast_reorg is set to OFF, ADD INDEX is executed as a transaction. If there are many update operations such as UPDATE and REPLACE in the target columns during the ADD INDEX execution, a larger batch size indicates a larger probability of transaction conflicts. In this case, it is recommended that you set the batch size to a smaller value. The minimum value is 32.
- If the transaction conflict does not exist, or if tidb_ddl_enable_fast_reorg is set to ON, you can set the batch size to a large value. This makes data backfilling faster but also increases the write pressure on TiKV. For a proper batch size, you also need to refer to the value of tidb_ddl_reorg_worker_cnt. See Interaction Test on Online Workloads and ADD INDEX Operations for reference.
- Starting from v8.3.0, this parameter is supported at the SESSION level. Modifying the parameter at the GLOBAL level will not impact currently running DDL statements. It will only apply to DDLs submitted in new sessions.

tidb_ddl_reorg_priority

Note

This variable is read-only for TiDB Serverless.

Scope: SESSION
Applies to hint SET_VAR: No
Type: Enumeration
Default value: PRIORITY_LOW
Value options: PRIORITY_LOW, PRIORITY_NORMAL, PRIORITY_HIGH
This variable is used to set the priority of executing the ADD INDEX operation in the re-organize phase.
You can set the value of this variable to PRIORITY_LOW, PRIORITY_NORMAL or PRIORITY_HIGH.

tidb_ddl_reorg_worker_cnt

Note

This variable is read-only for TiDB Serverless.

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 4
Range: [1, 256]
Unit: Threads
This variable is used to set the concurrency of the DDL operation in the re-organize phase.
Starting from v8.3.0, this parameter is supported at the SESSION level. Modifying the parameter at the GLOBAL level will not impact currently running DDL statements. It will only apply to DDLs submitted in new sessions.

`tidb_enable_fast_create_table` New in v8.0.0

Warning

This variable is currently an experimental feature and it is not recommended to use in a production environment. This feature might change or be removed without prior notice. If you find a bug, please give feedback by raising an issue on GitHub.

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: OFF
This variable is used to control whether to enable TiDB Accelerated Table Creation.
Starting from v8.0.0, TiDB supports accelerating table creation by the CREATE TABLE statement using tidb_enable_fast_create_table.
This variable is renamed from the variable tidb_ddl_version that is introduced in v7.6.0. Starting from v8.0.0, tidb_ddl_version no longer takes effect.

tidb_default_string_match_selectivity New in v6.2.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Float
Default value: 0.8
Range: [0, 1]
This variable is used to set the default selectivity of like, rlike, and regexp functions in the filter condition when estimating the number of rows. This variable also controls whether to enable TopN to help estimate these functions.
TiDB tries to estimate like in the filter condition using statistics. But when like matches a complex string, or when using rlike or regexp, TiDB often fails to fully use statistics, and the default value 0.8 is set as the selectivity rate instead, resulting in inaccurate estimation.
This variable is used to change the preceding behavior. If the variable is set to a value other than 0, the selectivity rate is the specified variable value instead of 0.8.
If the variable is set to 0, TiDB tries to evaluate using TopN in statistics to improve the accuracy and consider the NULL number in statistics when estimating the preceding three functions. The prerequisite is that statistics are collected when tidb_analyze_version is set to 2. Such evaluation might slightly affect the performance.
If the variable is set to a value other than the 0.8, TiDB adjusts the estimation for not like, not rlike, and not regexp accordingly.

tidb_disable_txn_auto_retry

Warning

Starting from v8.0.0, this variable is deprecated, and TiDB no longer supports automatic retries of optimistic transactions. As an alternative, when encountering optimistic transaction conflicts, you can capture the error and retry transactions in your application, or use the Pessimistic transaction mode instead.

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: ON
This variable is used to set whether to disable the automatic retry of explicit optimistic transactions. The default value of ON means that transactions will not automatically retry in TiDB and COMMIT statements might return errors that need to be handled in the application layer.
Setting the value to OFF means that TiDB will automatically retry transactions, resulting in fewer errors from COMMIT statements. Be careful when making this change, because it might result in lost updates.
This variable does not affect automatically committed implicit transactions and internally executed transactions in TiDB. The maximum retry count of these transactions is determined by the value of tidb_retry_limit.
For more details, see limits of retry.
This variable only applies to optimistic transactions, not to pessimistic transactions. The number of retries for pessimistic transactions is controlled by max_retry_count.

tidb_distsql_scan_concurrency

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Integer
Default value: 15
Range: [1, 256]
Unit: Threads
This variable is used to set the concurrency of the scan operation.
Use a bigger value in OLAP scenarios, and a smaller value in OLTP scenarios.
For OLAP scenarios, the maximum value should not exceed the number of CPU cores of all the TiKV nodes.
If a table has a lot of partitions, you can reduce the variable value appropriately (determined by the size of the data to be scanned and the frequency of the scan) to avoid TiKV becoming out of memory (OOM).
For a simple query with only a LIMIT clause, if the LIMIT value is less than 100000, the scan operation pushed down to TiKV treats the value of this variable as 1 to enhance execution efficiency.
For the SELECT MAX/MIN(col) FROM ... query, if the col column has an index sorted in the same order required by the MAX(col) or MIN(col) function, TiDB will rewrite the query to SELECT col FROM ... LIMIT 1 for processing, and the value of this variable will also be processed as 1. For example, for SELECT MIN(col) FROM ..., if the col column has an ascending index, TiDB can quickly obtain the MIN(col) value by rewriting the query to SELECT col FROM ... LIMIT 1 and directly reading the first row of the index.

tidb_dml_batch_size

Warning

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 0
Range: [0, 2147483647]
Unit: Rows
When this value is greater than 0, TiDB will batch commit statements such as INSERT into smaller transactions. This reduces memory usage and helps ensure that the txn-total-size-limit is not reached by bulk modifications.
Only the value 0 provides ACID compliance. Setting this to any other value will break the atomicity and isolation guarantees of TiDB.
To make this variable work, you also need to enable tidb_enable_batch_dml and at least one of tidb_batch_insert and tidb_batch_delete.

Note

Starting from v7.0.0, tidb_dml_batch_size no longer takes effect on the LOAD DATA statement.

tidb_dml_type New in v8.0.0

Warning

The bulk DML execution mode (tidb_dml_type = "bulk") is an experimental feature. It is not recommended that you use it in the production environment. This feature might be changed or removed without prior notice. If you find a bug, you can report an issue. In the current version, when TiDB performs large transactions using the bulk DML mode, it might affect the memory usage and execution efficiency of TiCDC, TiFlash, and the resolved-ts module of TiKV, and might cause OOM issues. Additionally, BR might be blocked and fail to process when encountering locks. Therefore, it is not recommended to use this mode when these components or features are enabled.

Scope: SESSION
Applies to hint SET_VAR: Yes
Type: String
Default value: "standard"
Value options: "standard", "bulk"
This variable controls the execution mode of DML statements.
- "standard" indicates the standard DML execution mode, where TiDB transactions are cached in memory before being committed. This mode is suitable for high-concurrency transaction scenarios with potential conflicts and is the default recommended execution mode.
- "bulk" indicates the bulk DML execution mode, which is suitable for scenarios where a large amount of data is written, causing excessive memory usage in TiDB.
  - During the execution of TiDB transactions, the data is not fully cached in the TiDB memory, but is continuously written to TiKV to reduce memory usage and smooth the write pressure.
  - Only INSERT, UPDATE, REPLACE, and DELETE statements are affected by the "bulk" mode. Due to the pipelined execution in "bulk" mode, the usage of INSERT IGNORE ... ON DUPLICATE UPDATE ... might result in a Duplicate entry error when updates cause conflicts. In contrast, in "standard" mode, because the IGNORE keyword is set, this error would be ignored and not be returned to the user.
  - "bulk" mode is only suitable for scenarios where a large amount of data is written without conflicts. This mode is not efficient for handling write conflicts, as write-write conflicts might cause large transactions to fail and be rolled back.
  - "bulk" mode only takes effect on statements with auto-commit enabled, and requires the pessimistic-auto-commit configuration item to be set to false.
  - When using the "bulk" mode to execute statements, ensure that the metadata lock remains enabled during the execution process.
  - "bulk" mode cannot be used on temporary tables and cached tables.
  - "bulk" mode cannot be used on tables containing foreign keys and tables referenced by foreign keys when the foreign key constraint check is enabled (foreign_key_checks = ON).
  - In situations that the environment does not support or is incompatible with the "bulk" mode, TiDB falls back to the "standard" mode and returns a warning message. To verify if the "bulk" mode is used, you can check the pipelined field using tidb_last_txn_info. A true value indicates that the "bulk" mode is used.
  - When executing large transactions in the "bulk" mode, the transaction duration might be long. For transactions in this mode, the maximum TTL of the transaction lock is the greater value between max-txn-ttl and 24 hours. Additionally, if the transaction execution time exceeds the value set by tidb_gc_max_wait_time, the GC might force a rollback of the transaction, leading to its failure.
  - When TiDB executes transactions in the "bulk" mode, transaction size is not limited by the TiDB configuration item txn-total-size-limit.
  - This mode is implemented by the Pipelined DML feature. For detailed design and GitHub issues, see Pipelined DML and #50215.

tidb_enable_1pc New in v5.0

Note

This variable is read-only for TiDB Serverless.

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: ON
This variable is used to specify whether to enable the one-phase commit feature for transactions that only affect one Region. Compared with the often-used two-phase commit, one-phase commit can greatly reduce the latency of transaction commit and increase the throughput.

Note

The default value of ON only applies to new clusters. if your cluster was upgraded from an earlier version of TiDB, the value OFF will be used instead.
If you have enabled TiDB Binlog (deprecated), enabling this variable cannot improve the performance. To improve the performance, it is recommended to use TiCDC instead.
Enabling this parameter only means that one-phase commit becomes an optional mode of transaction commit. In fact, the most suitable mode of transaction commit is determined by TiDB.

tidb_enable_analyze_snapshot New in v6.2.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: OFF
This variable controls whether to read historical data or the latest data when performing ANALYZE. If this variable is set to ON, ANALYZE reads the historical data available at the time of ANALYZE. If this variable is set to OFF, ANALYZE reads the latest data.
Before v5.2, ANALYZE reads the latest data. From v5.2 to v6.1, ANALYZE reads the historical data available at the time of ANALYZE.

Warning

If ANALYZE reads the historical data available at the time of ANALYZE, the long duration of AUTO ANALYZE might cause the GC life time is shorter than transaction duration error because the historical data is garbage-collected.

tidb_enable_async_commit New in v5.0

Note

This variable is read-only for TiDB Serverless.

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: ON
This variable controls whether to enable the async commit feature for the second phase of the two-phase transaction commit to perform asynchronously in the background. Enabling this feature can reduce the latency of transaction commit.

Note

The default value of ON only applies to new clusters. if your cluster was upgraded from an earlier version of TiDB, the value OFF will be used instead.
If you have enabled TiDB Binlog (deprecated), enabling this variable cannot improve the performance. To improve the performance, it is recommended to use TiCDC instead.
Enabling this parameter only means that Async Commit becomes an optional mode of transaction commit. In fact, the most suitable mode of transaction commit is determined by TiDB.

tidb_enable_auto_analyze New in v6.1.0

Note

This variable is read-only for TiDB Serverless.

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: ON
Determines whether TiDB automatically updates table statistics as a background operation.
This setting was previously a tidb.toml option (performance.run-auto-analyze), but changed to a system variable starting from TiDB v6.1.0.

tidb_enable_auto_analyze_priority_queue New in v8.0.0

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: ON
This variable is used to control whether to enable the priority queue to schedule the tasks of automatically collecting statistics. When this variable is enabled, TiDB prioritizes collecting statistics for tables that are more valuable to collect, such as newly created indexes and partitioned tables with partition changes. Additionally, TiDB prioritizes tables with lower health scores, placing them at the front of the queue.

tidb_enable_auto_increment_in_generated

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: OFF
This variable is used to determine whether to include the AUTO_INCREMENT columns when creating a generated column or an expression index.

tidb_enable_batch_dml

Warning

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: OFF
This variable controls whether to enable the deprecated batch-dml feature. When it is enabled, certain statements might be split into multiple transactions, which is non-atomic and should be used with care. When using batch-dml, you must ensure that there are no concurrent operations on the data you are operating on. To make it work, you must also specify a positive value for tidb_batch_dml_size and enable at least one of tidb_batch_insert and tidb_batch_delete.

tidb_enable_cascades_planner

Warning

Currently, cascades planner is an experimental feature. It is not recommended that you use it in production environments.

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Boolean
Default value: OFF
This variable is used to control whether to enable the cascades planner.

tidb_enable_check_constraint New in v7.2.0

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: OFF
This variable is used to control whether to enable the CHECK constraint feature.

tidb_enable_chunk_rpc New in v4.0

Scope: SESSION
Applies to hint SET_VAR: No
Type: Boolean
Default value: ON
This variable is used to control whether to enable the Chunk data encoding format in Coprocessor.

tidb_enable_clustered_index New in v5.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Enumeration
Default value: ON
Possible values: OFF, ON, INT_ONLY
This variable is used to control whether to create the primary key as a clustered index by default. "By default" here means that the statement does not explicitly specify the keyword CLUSTERED/NONCLUSTERED. Supported values are OFF, ON, and INT_ONLY:
- OFF indicates that primary keys are created as non-clustered indexes by default.
- ON indicates that primary keys are created as clustered indexes by default.
- INT_ONLY indicates that the behavior is controlled by the configuration item alter-primary-key. If alter-primary-key is set to true, all primary keys are created as non-clustered indexes by default. If it is set to false, only the primary keys which consist of an integer column are created as clustered indexes.

tidb_enable_ddl New in v6.3.0

Note

This variable is read-only for TiDB Serverless.

Scope: GLOBAL
Persists to cluster: No, only applicable to the current TiDB instance that you are connecting to.
Applies to hint SET_VAR: No
Default value: ON
Possible values: OFF, ON
This variable controls whether the corresponding TiDB instance can become a DDL owner or not. If there is only one TiDB instance in the current TiDB cluster, you cannot prevent it from becoming a DDL owner, which means you cannot set it to OFF.

tidb_enable_collect_execution_info

Note

This TiDB variable is not applicable to TiDB Cloud.

Scope: GLOBAL
Persists to cluster: No, only applicable to the current TiDB instance that you are connecting to.
Applies to hint SET_VAR: No
Type: Boolean
Default value: ON
This variable controls whether to record the execution information of each operator in the slow query log and whether to record the usage statistics of indexes.

tidb_enable_column_tracking New in v5.4.0

Warning

Starting from v8.3.0, this variable is deprecated. TiDB tracks predicate columns by default. For more information, see tidb_analyze_column_options.

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: OFF
This variable controls whether to enable TiDB to collect PREDICATE COLUMNS. After enabling the collection, if you disable it, the information of previously collected PREDICATE COLUMNS is cleared. For details, see Collect statistics on some columns.

tidb_enable_enhanced_security

Scope: NONE
Applies to hint SET_VAR: No
Type: Boolean

Default value: OFF
This variable indicates whether the TiDB server you are connected to has the Security Enhanced Mode (SEM) enabled. To change its value, you need to modify the value of enable-sem in your TiDB server configuration file and restart the TiDB server.

SEM is inspired by the design of systems such as Security-Enhanced Linux. It reduces the abilities of users with the MySQL SUPER privilege and instead requires RESTRICTED fine-grained privileges to be granted as a replacement. These fine-grained privileges include:
- RESTRICTED_TABLES_ADMIN: The ability to write data to system tables in the mysql schema and to see sensitive columns on information_schema tables.
- RESTRICTED_STATUS_ADMIN: The ability to see sensitive variables in the command SHOW STATUS.
- RESTRICTED_VARIABLES_ADMIN: The ability to see and set sensitive variables in SHOW [GLOBAL] VARIABLES and SET.
- RESTRICTED_USER_ADMIN: The ability to prevent other users from making changes or dropping a user account.

tidb_enable_exchange_partition

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: ON
This variable controls whether to enable the exchange partitions with tables feature. The default value is ON, that is, exchange partitions with tables is enabled by default.
This variable is deprecated since v6.3.0. Its value will be fixed to the default value ON, that is, exchange partitions with tables is enabled by default.

tidb_enable_extended_stats

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Boolean
Default value: OFF
This variable indicates whether TiDB can collect the extended statistic to guide the optimizer. See Introduction to Extended Statistics for more information.

tidb_enable_external_ts_read New in v6.4.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: OFF
If this variable is set to ON, TiDB reads data with the timestamp specified by tidb_external_ts.

tidb_external_ts New in v6.4.0

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 0
If tidb_enable_external_ts_read is set to ON, TiDB reads data with the timestamp specified by this variable.

tidb_enable_fast_analyze

Warning

Starting from v7.5.0, this variable is deprecated.

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: OFF
This variable is used to set whether to enable the statistics Fast Analyze feature.
If the statistics Fast Analyze feature is enabled, TiDB randomly samples about 10,000 rows of data as statistics. When the data is distributed unevenly or the data size is small, the statistics accuracy is low. This might lead to a non-optimal execution plan, for example, selecting a wrong index. If the execution time of the regular Analyze statement is acceptable, it is recommended to disable the Fast Analyze feature.

tidb_enable_fast_table_check New in v7.2.0

Note

This variable does not work for multi-valued indexes and prefix indexes.

Scope: SESSION | GLOBAL
Persists to the cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: ON
This variable is used to control whether to use a checksum-based approach to quickly check the integrity of data and indexes in a table. The default value ON means this feature is enabled by default.
When this variable is enabled, TiDB can execute the ADMIN CHECK [TABLE|INDEX] statement in a faster way.

tidb_enable_foreign_key New in v6.3.0

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: Before v6.6.0, the default value is OFF. Starting from v6.6.0, the default value is ON.
This variable controls whether to enable the FOREIGN KEY feature.

tidb_enable_gc_aware_memory_track

Warning

This variable is an internal variable for debugging in TiDB. It might be removed in a future release. Do not set this variable.

Note

This variable is read-only for TiDB Serverless.

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: OFF
This variable controls whether to enable GC-Aware memory track.

tidb_enable_global_index New in v7.6.0

Warning

The feature controlled by this variable is experimental. It is not recommended that you use it in the production environment. This feature might be changed or removed without prior notice. If you find a bug, you can report an issue on GitHub.

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: OFF
Possible values: OFF, ON
This variable controls whether to support creating global indexes for partitioned tables. When this variable is enabled, TiDB allows you to create unique indexes that do not include all the columns used in the partition expressions by specifying GLOBAL in the index definition.

tidb_enable_lazy_cursor_fetch New in v8.3.0

Warning

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: OFF
Possible values: OFF, ON
This variable controls the behavior of the Cursor Fetch feature.
- When Cursor Fetch is enabled and this variable is set to OFF, TiDB reads all the data at the start of statement execution, stores the data into TiDB's memory, and returns it to the client based on the client's specified FetchSize for subsequent client reads. If the result set is too large, TiDB might temporarily write the result to the hard disk.
- When Cursor Fetch is enabled and this variable is set to ON, TiDB does not read all the data into the TiDB node at once, but reads data into the TiDB node incrementally as the client fetches it.
The feature controlled by this variable has the following limitations:
- It does not support statements within explicit transactions.
- It only supports execution plans that contain and only contain TableReader, IndexReader, IndexLookUp, Projection, and Selection operators.
- For statements using Lazy Cursor Fetch, execution information does not appear in the statements summary and slow query log.
For unsupported scenarios, its behavior is the same as when setting this variable to OFF.

tidb_enable_non_prepared_plan_cache

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Boolean
Default value: OFF
This variable controls whether to enable the Non-prepared plan cache feature.
Enabling this feature might incur additional memory and CPU overhead and might not be suitable for all situations. Please determine whether to enable this feature according to your actual scenario.

tidb_enable_non_prepared_plan_cache_for_dml New in v7.1.0

Warning

The non-prepared execution plan cache for DML statements is an experimental feature. It is not recommended that you use it in the production environment. This feature might be changed or removed without prior notice. If you find a bug, you can report an issue on GitHub.

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: OFF.
This variable controls whether to enable the Non-prepared plan cache feature for DML statements.

tidb_enable_gogc_tuner New in v6.4.0

Note

This variable is read-only for TiDB Serverless.

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: ON
This variable controls whether to enable GOGC Tuner.

tidb_enable_historical_stats

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: OFF. Before v8.2.0, the default value is ON.
This variable controls whether to enable historical statistics. The default value is OFF, which means that historical statistics are disabled by default.

tidb_enable_historical_stats_for_capture

Warning

The feature controlled by this variable is not fully functional in the current TiDB version. Do not change the default value.

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: OFF
This variable controls whether the information captured by PLAN REPLAYER CAPTURE includes historical statistics by default. The default value OFF means that historical statistics are not included by default.

tidb_enable_index_merge New in v4.0

Note

After upgrading a TiDB cluster from versions earlier than v4.0.0 to v5.4.0 or later, this variable is disabled by default to prevent performance regression due to changes of execution plans.
After upgrading a TiDB cluster from v4.0.0 or later to v5.4.0 or later, this variable remains the setting before the upgrade.
Since v5.4.0, for a newly deployed TiDB cluster, this variable is enabled by default.

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Boolean
Default value: ON
This variable is used to control whether to enable the index merge feature.

tidb_enable_index_merge_join

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Boolean
Default value: OFF
Specifies whether to enable the IndexMergeJoin operator.
This variable is used only for the internal operation of TiDB. It is NOT recommended to adjust it. Otherwise, data correctness might be affected.

tidb_enable_legacy_instance_scope New in v6.0.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: ON
This variable permits INSTANCE scoped variables to be set using the SET SESSION as well as SET GLOBAL syntax.
This option is enabled by default for compatibility with earlier versions of TiDB.

tidb_enable_list_partition New in v5.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: ON
This variable is used to set whether to enable the LIST (COLUMNS) TABLE PARTITION feature.

tidb_enable_local_txn

Note

This variable is read-only for TiDB Serverless.

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: OFF
This variable is used for an unreleased feature. Do not change the variable value.

tidb_enable_metadata_lock New in v6.3.0

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: ON
This variable is used to set whether to enable the Metadata lock feature. Note that when setting this variable, you need to make sure that there are no running DDL statements in the cluster. Otherwise, the data might be incorrect or inconsistent.

tidb_enable_mutation_checker New in v6.0.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: ON
This variable is used to control whether to enable TiDB mutation checker, which is a tool used to check consistency between data and indexes during the execution of DML statements. If the checker returns an error for a statement, TiDB rolls back the execution of the statement. Enabling this variable causes a slight increase in CPU usage. For more information, see Troubleshoot Inconsistency Between Data and Indexes.
For new clusters of v6.0.0 or later versions, the default value is ON. For existing clusters that upgrade from versions earlier than v6.0.0, the default value is OFF.

tidb_enable_new_cost_interface New in v6.2.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: ON
TiDB v6.2.0 refactors the implementation of previous cost model. This variable controls whether to enable the refactored Cost Model implementation.
This variable is enabled by default because the refactored Cost Model uses the same cost formula as before, which does not change the plan decision.
If your cluster is upgraded from v6.1 to v6.2, this variable remains OFF, and it is recommended to enable it manually. If your cluster is upgraded from a version earlier than v6.1, this variable sets to ON by default.

tidb_enable_new_only_full_group_by_check New in v6.1.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Boolean
Default value: OFF
This variable controls the behavior when TiDB performs the ONLY_FULL_GROUP_BY check. For detailed information about ONLY_FULL_GROUP_BY, see the MySQL documentation. In v6.1.0, TiDB handles this check more strictly and correctly.
To avoid potential compatibility issues caused by version upgrades, the default value of this variable is OFF in v6.1.0.

tidb_enable_noop_functions New in v4.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Enumeration
Default value: OFF
Possible values: OFF, ON, WARN
By default, TiDB returns an error when you attempt to use the syntax for functionality that is not yet implemented. When the variable value is set to ON, TiDB silently ignores such cases of unavailable functionality, which is helpful if you cannot make changes to the SQL code.
Enabling noop functions controls the following behaviors:
- LOCK IN SHARE MODE syntax
- SQL_CALC_FOUND_ROWS syntax
- START TRANSACTION READ ONLY and SET TRANSACTION READ ONLY syntax
- The tx_read_only, transaction_read_only, offline_mode, super_read_only, read_only and sql_auto_is_null system variables
- GROUP BY <expr> ASC|DESC syntax

Warning

Only the default value of OFF can be considered safe. Setting tidb_enable_noop_functions=1 might lead to unexpected behaviors in your application, because it permits TiDB to ignore certain syntax without providing an error. For example, the syntax START TRANSACTION READ ONLY is permitted, but the transaction remains in read-write mode.

tidb_enable_noop_variables New in v6.2.0

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Default value: ON
If you set the variable value to OFF, TiDB behaves as follows:
- When you use SET to set a noop variable, TiDB returns the "setting *variable_name* has no effect in TiDB" warning.
- The result of SHOW [SESSION | GLOBAL] VARIABLES does not include noop variables.
- When you use SELECT to read a noop variable, TiDB returns the "variable *variable_name* has no effect in TiDB" warning.
To check whether a TiDB instance has set and read the noop variable, you can use the SELECT * FROM INFORMATION_SCHEMA.CLIENT_ERRORS_SUMMARY_GLOBAL; statement.

tidb_enable_null_aware_anti_join New in v6.3.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Default value: Before v7.0.0, the default value is OFF. Starting from v7.0.0, the default value is ON.
Type: Boolean
This variable controls whether TiDB applies Null Aware Hash Join when ANTI JOIN is generated by subqueries led by special set operators NOT IN and != ALL.
When you upgrade from an earlier version to a v7.0.0 or later cluster, the feature is automatically enabled, meaning that this variable is set to ON.

tidb_enable_outer_join_reorder New in v6.1.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Boolean
Default value: ON
Since v6.1.0, the Join Reorder algorithm of TiDB supports Outer Join. This variable controls whether TiDB enables the Join Reorder's support for Outer Join.
If your cluster is upgraded from an earlier version of TiDB, note the following:
- If the TiDB version before the upgrade is earlier than v6.1.0, the default value of this variable after the upgrade is ON.
- If the TiDB version before the upgrade is v6.1.0 or later, the default value of the variable after the upgrade follows the value before the upgrade.

`tidb_enable_inl_join_inner_multi_pattern` New in v7.0.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Boolean
Default value: OFF
This variable controls whether Index Join is supported when the inner table has Selection or Projection operators on it. The default value OFF means that Index Join is not supported in this scenario.

tidb_enable_ordered_result_mode

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Boolean
Default value: OFF
Specifies whether to sort the final output result automatically.
For example, with this variable enabled, TiDB processes SELECT a, MAX(b) FROM t GROUP BY a as SELECT a, MAX(b) FROM t GROUP BY a ORDER BY a, MAX(b).

tidb_enable_paging New in v5.4.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Boolean
Default value: ON
This variable controls whether to use the method of paging to send coprocessor requests. For TiDB versions in [v5.4.0, v6.2.0), this variable only takes effect on the IndexLookup operator; for v6.2.0 and later, this variable takes effect globally. Starting from v6.4.0, the default value of this variable is changed from OFF to ON.
User scenarios:
- In all OLTP scenarios, it is recommended to use the method of paging.
- For read queries that use IndexLookup and Limit and that Limit cannot be pushed down to IndexScan, there might be high latency for the read queries and high usage for TiKV Unified read pool CPU. In such cases, because the Limit operator only requires a small set of data, if you set tidb_enable_paging to ON, TiDB processes less data, which reduces query latency and resource consumption.
- In scenarios such as data export using Dumpling and full table scan, enabling paging can effectively reduce the memory consumption of TiDB processes.

Note

In OLAP scenarios where TiKV is used as the storage engine instead of TiFlash, enabling paging might cause performance regression in some cases. If the regression occurs, consider using this variable to disable paging, or using the tidb_min_paging_size and tidb_max_paging_size variables to adjust the range of rows for paging size.

tidb_enable_parallel_apply New in v5.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: OFF
This variable controls whether to enable concurrency for the Apply operator. The number of concurrencies is controlled by the tidb_executor_concurrency variable. The Apply operator processes correlated subqueries and has no concurrency by default, so the execution speed is slow. Setting this variable value to 1 can increase concurrency and speed up execution. Currently, concurrency for Apply is disabled by default.

tidb_enable_parallel_hashagg_spill New in v8.0.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: ON
This variable controls whether TiDB supports disk spill for the parallel HashAgg algorithm. When it is ON, the HashAgg operator can automatically trigger data spill based on memory usage under any parallel conditions, thus balancing performance and data throughput. It is not recommended to set this variable to OFF. Starting from v8.2.0, setting it to OFF will report an error. This variable will be deprecated in a future release.

tidb_enable_pipelined_window_function

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: ON
This variable specifies whether to use the pipeline execution algorithm for window functions.

tidb_enable_plan_cache_for_param_limit New in v6.6.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: ON
This variable controls whether Prepared Plan Cache caches execution plans with a variable as the LIMIT parameter (LIMIT ?). The default value is ON, which means Prepared Plan Cache supports caching such execution plans. Note that Prepared Plan Cache does not support caching execution plans with a variable that is greater than 10000.

tidb_enable_plan_cache_for_subquery New in v7.0.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: ON
This variable controls whether Prepared Plan Cache caches queries that contain subqueries.

tidb_enable_plan_replayer_capture

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: ON
This variable controls whether to enable the PLAN REPLAYER CAPTURE feature. The default value ON means to enable the PLAN REPLAYER CAPTURE feature.

tidb_enable_plan_replayer_continuous_capture New in v7.0.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: OFF
This variable controls whether to enable the PLAN REPLAYER CONTINUOUS CAPTURE feature. The default value OFF means to disable the feature.

tidb_enable_prepared_plan_cache New in v6.1.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Boolean
Default value: ON
Determines whether to enable Prepared Plan Cache. When it is enabled, the execution plans of Prepare and Execute are cached so that the subsequent executions skip optimizing the execution plans, which brings performance improvement.
This setting was previously a tidb.toml option (prepared-plan-cache.enabled), but changed to a system variable starting from TiDB v6.1.0.

tidb_enable_prepared_plan_cache_memory_monitor New in v6.4.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Default value: ON
This variable controls whether to count the memory consumed by the execution plans cached in the Prepared Plan Cache. For details, see Memory management of Prepared Plan Cache.

tidb_enable_pseudo_for_outdated_stats New in v5.3.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Boolean
Default value: OFF
This variable controls the behavior of the optimizer on using statistics of a table when the statistics are outdated.

The optimizer determines whether the statistics of a table is outdated in this way: since the last time ANALYZE is executed on a table to get the statistics, if 80% of the table rows are modified (the modified row count divided by the total row count), the optimizer determines that the statistics of this table is outdated. You can change this ratio using the pseudo-estimate-ratio configuration.

By default (with the variable value OFF), when the statistics of a table is outdated, the optimizer still keeps using the statistics of the table. If you set the variable value to ON, the optimizer determines that the statistics of the table is no longer reliable except for the total row count. Then, the optimizer uses the pseudo statistics.
If the data on a table is frequently modified without executing ANALYZE on this table in time, to keep the execution plan stable, it is recommended to set the variable value to OFF.

tidb_enable_rate_limit_action

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: OFF
This variable controls whether to enable the dynamic memory control feature for the operator that reads data. By default, this operator enables the maximum number of threads that tidb_distsql_scan_concurrency allows to read data. When the memory usage of a single SQL statement exceeds tidb_mem_quota_query each time, the operator that reads data stops one thread.

When the operator that reads data has only one thread left and the memory usage of a single SQL statement constantly exceeds tidb_mem_quota_query, this SQL statement triggers other memory control behaviors, such as spilling data to disk.
This variable controls memory usage effectively when an SQL statement only reads data. If computing operations (such as join or aggregation operations) are required, memory usage might not be under the control of tidb_mem_quota_query, which increases the risk of OOM.

tidb_enable_resource_control New in v6.6.0

Note

This variable is read-only for TiDB Serverless.

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Default value: ON
Type: Boolean
This variable is a switch for the resource control feature. When this variable is set to ON, the TiDB cluster can isolate application resources based on resource groups.

tidb_enable_reuse_chunk New in v6.4.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Default value: ON
Value options: OFF, ON
This variable controls whether TiDB enables chunk objects cache. If the value is ON, TiDB prefers to use the cached chunk object and only requests from the system if the requested object is not in the cache. If the value is OFF, TiDB requests chunk objects from the system directly.

tidb_enable_shared_lock_promotion New in v8.3.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: OFF
This variable controls whether to enable the feature of upgrading shared locks to exclusive locks. TiDB does not support SELECT LOCK IN SHARE MODE by default. When the variable value is ON, TiDB tries to upgrade the SELECT LOCK IN SHARE MODE statement to SELECT FOR UPDATE and add a pessimistic lock. The default value of this variable is OFF, which means that the feature of upgrading shared locks to exclusive locks is disabled.
Enabling this variable takes effect on the SELECT LOCK IN SHARE MODE statement, regardless of whether tidb_enable_noop_functions is enabled or not.

tidb_enable_slow_log

Note

This TiDB variable is not applicable to TiDB Cloud.

Scope: GLOBAL
Persists to cluster: No, only applicable to the current TiDB instance that you are connecting to.
Applies to hint SET_VAR: No
Type: Boolean
Default value: ON
This variable is used to control whether to enable the slow log feature.

tidb_enable_tmp_storage_on_oom

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Default value: ON
Value options: OFF, ON
Controls whether to enable the temporary storage for some operators when a single SQL statement exceeds the memory quota specified by the system variable tidb_mem_quota_query.
Before v6.3.0, you can enable or disable this feature by using the TiDB configuration item oom-use-tmp-storage. After upgrading the cluster to v6.3.0 or a later version, the TiDB cluster will initialize this variable using the value of oom-use-tmp-storage automatically. After that, changing the value of oom-use-tmp-storage does not take effect anymore.

tidb_enable_stmt_summary New in v3.0.4

Note

This variable is read-only for TiDB Serverless.

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: ON
This variable is used to control whether to enable the statement summary feature. If enabled, SQL execution information like time consumption is recorded to the information_schema.STATEMENTS_SUMMARY system table to identify and troubleshoot SQL performance issues.

tidb_enable_strict_double_type_check New in v5.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: ON
This variable is used to control if tables can be created with invalid definitions of type DOUBLE. This setting is intended to provide an upgrade path from earlier versions of TiDB, which were less strict in validating types.
The default value of ON is compatible with MySQL.

For example, the type DOUBLE(10) is now considered invalid because the precision of floating point types is not guaranteed. After changing tidb_enable_strict_double_type_check to OFF, the table is created:

mysql> CREATE TABLE t1 (id int, c double(10));
ERROR 1149 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use

mysql> SET tidb_enable_strict_double_type_check = 'OFF';
Query OK, 0 rows affected (0.00 sec)

mysql> CREATE TABLE t1 (id int, c double(10));
Query OK, 0 rows affected (0.09 sec)

Note

This setting only applies to the type DOUBLE since MySQL permits precision for FLOAT types. This behavior is deprecated starting with MySQL 8.0.17, and it is not recommended to specify precision for either FLOAT or DOUBLE types.

tidb_enable_table_partition

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Enumeration
Default value: ON
Possible values: OFF, ON, AUTO
This variable is used to set whether to enable the TABLE PARTITION feature:
- ON indicates enabling Range partitioning, Hash partitioning, and Range column partitioning with one single column.
- AUTO functions the same way as ON does.
- OFF indicates disabling the TABLE PARTITION feature. In this case, the syntax that creates a partition table can be executed, but the table created is not a partitioned one.

tidb_enable_telemetry New in v4.0.2 and deprecated in v8.1.0

Warning

Starting from v8.1.0, the telemetry feature in TiDB is removed, and this variable is no longer functional. It is retained solely for compatibility with earlier versions.

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: OFF

Before v8.1.0, this variable controls whether to enable the telemetry collection in TiDB.

tidb_enable_tiflash_read_for_write_stmt New in v6.3.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: ON
This variable controls whether read operations in SQL statements containing INSERT, DELETE, and UPDATE can be pushed down to TiFlash. For example:
- SELECT queries in INSERT INTO SELECT statements (typical usage scenario: TiFlash query result materialization)
- WHERE condition filtering in UPDATE and DELETE statements
Starting from v7.1.0, this variable is deprecated. When tidb_allow_mpp = ON, the optimizer intelligently decides whether to push a query down to TiFlash based on the SQL mode and the cost estimates of the TiFlash replica. Note that TiDB allows read operations in SQL statements containing INSERT, DELETE, and UPDATE (such as INSERT INTO SELECT) to be pushed down to TiFlash only when the SQL Mode of the current session is not strict, which means that the sql_mode value does not contain STRICT_TRANS_TABLES and STRICT_ALL_TABLES.

tidb_enable_top_sql New in v5.4.0

Note

This TiDB variable is not applicable to TiDB Cloud.

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: OFF

This variable is used to control whether to enable the Top SQL feature.

tidb_enable_tso_follower_proxy New in v5.3.0

Note

This variable is read-only for TiDB Serverless.

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: OFF
This variable controls whether to enable the TSO Follower Proxy feature. When the value is OFF, TiDB only gets TSO from the PD leader. When the value is ON, TiDB evenly distributes requests for TSO to all PD servers, and PD followers can also handle TSO requests, thereby reducing the CPU pressure on the PD leader.
Scenarios for enabling TSO Follower Proxy:
- Due to the high pressure of TSO requests, the CPU of the PD leader reaches a bottleneck, which causes high latency of TSO RPC requests.
- The TiDB cluster has many TiDB instances, and increasing the value of tidb_tso_client_batch_max_wait_time cannot alleviate the high latency issue of TSO RPC requests.

Note

Suppose that the TSO RPC latency increases for reasons other than a CPU usage bottleneck of the PD leader (such as network issues). In this case, enabling the TSO Follower Proxy might increase the execution latency in TiDB and affect the QPS performance of the cluster.

tidb_enable_unsafe_substitute New in v6.3.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: OFF
This variable controls whether to replace expressions with generated columns in an unsafe way. The default value is OFF, which means that unsafe replacement is disabled by default. For more details, see Generated Columns.

tidb_enable_vectorized_expression New in v4.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Boolean
Default value: ON
This variable is used to control whether to enable vectorized execution.

tidb_enable_window_function

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: ON
This variable is used to control whether to enable the support for window functions. Note that window functions might use reserved keywords. This might cause SQL statements that can be executed normally to fail to be parsed after TiDB is upgraded. In this case, you can set tidb_enable_window_function to OFF.

`tidb_enable_row_level_checksum` New in v7.1.0

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: OFF

This variable is used to control whether to enable the TiCDC data integrity validation for single-row data feature.

You can use the TIDB_ROW_CHECKSUM() function to get the checksum value of a row.

tidb_enforce_mpp New in v5.1

Scope: SESSION
Applies to hint SET_VAR: Yes
Type: Boolean
Default value: OFF

To change this default value, modify the performance.enforce-mpp configuration value.

Controls whether to ignore the optimizer's cost estimation and to forcibly use TiFlash's MPP mode for query execution. The value options are as follows:
- 0 or OFF, which means that the MPP mode is not forcibly used (by default).
- 1 or ON, which means that the cost estimation is ignored and the MPP mode is forcibly used. Note that this setting only takes effect when tidb_allow_mpp=true.

tidb_evolve_plan_baselines New in v4.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: OFF
This variable is used to control whether to enable the baseline evolution feature. For detailed introduction or usage , see Baseline Evolution.
To reduce the impact of baseline evolution on the cluster, use the following configurations:
- Set tidb_evolve_plan_task_max_time to limit the maximum execution time of each execution plan. The default value is 600s.
- Set tidb_evolve_plan_task_start_time and tidb_evolve_plan_task_end_time to limit the time window. The default values are respectively 00:00 +0000 and 23:59 +0000.

tidb_evolve_plan_task_end_time New in v4.0

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Time
Default value: 23:59 +0000
This variable is used to set the end time of baseline evolution in a day.

tidb_evolve_plan_task_max_time New in v4.0

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 600
Range: [-1, 9223372036854775807]
Unit: Seconds
This variable is used to limit the maximum execution time of each execution plan in the baseline evolution feature.

tidb_evolve_plan_task_start_time New in v4.0

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Time
Default value: 00:00 +0000
This variable is used to set the start time of baseline evolution in a day.

tidb_executor_concurrency New in v5.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Integer
Default value: 5
Range: [1, 256]
Unit: Threads

This variable is used to set the concurrency of the following SQL operators (to one value):

index lookup
index lookup join
hash join
hash aggregation (the partial and final phases)
window
projection
sort

tidb_executor_concurrency incorporates the following existing system variables as a whole for easier management:

tidb_index_lookup_concurrency
tidb_index_lookup_join_concurrency
tidb_hash_join_concurrency
tidb_hashagg_partial_concurrency
tidb_hashagg_final_concurrency
tidb_projection_concurrency
tidb_window_concurrency

Since v5.0, you can still separately modify the system variables listed above (with a deprecation warning returned) and your modification only affects the corresponding single operators. After that, if you use tidb_executor_concurrency to modify the operator concurrency, the separately modified operators will not be affected. If you want to use tidb_executor_concurrency to modify the concurrency of all operators, you can set the values of all variables listed above to -1.

For a system upgraded to v5.0 from an earlier version, if you have not modified any value of the variables listed above (which means that the tidb_hash_join_concurrency value is 5 and the values of the rest are 4), the operator concurrency previously managed by these variables will automatically be managed by tidb_executor_concurrency. If you have modified any of these variables, the concurrency of the corresponding operators will still be controlled by the modified variables.

tidb_expensive_query_time_threshold

Note

This TiDB variable is not applicable to TiDB Cloud.

Scope: GLOBAL
Persists to cluster: No, only applicable to the current TiDB instance that you are connecting to.
Applies to hint SET_VAR: No
Type: Integer
Default value: 60
Range: [10, 2147483647]
Unit: Seconds
This variable is used to set the threshold value that determines whether to print expensive query logs. The difference between expensive query logs and slow query logs is:
- Slow logs are printed after the statement is executed.
- Expensive query logs print the statements that are being executed, with execution time exceeding the threshold value, and their related information.

tidb_expensive_txn_time_threshold New in v7.2.0

Scope: GLOBAL
Persists to cluster: No, only applicable to the current TiDB instance that you are connecting to.
Applies to hint SET_VAR: No
Type: Integer
Default value: 600
Range: [60, 2147483647]
Unit: Seconds
This variable controls the threshold for logging expensive transactions, which is 600 seconds by default. When the duration of a transaction exceeds the threshold, and the transaction is neither committed nor rolled back, it is considered an expensive transaction and will be logged.

tidb_force_priority

Note

This TiDB variable is not applicable to TiDB Cloud.

Scope: GLOBAL
Persists to cluster: No, only applicable to the current TiDB instance that you are connecting to.
Applies to hint SET_VAR: No
Type: Enumeration
Default value: NO_PRIORITY
Possible values: NO_PRIORITY, LOW_PRIORITY, HIGH_PRIORITY, DELAYED
This variable is used to change the default priority for statements executed on a TiDB server. A use case is to ensure that a particular user that is performing OLAP queries receives lower priority than users performing OLTP queries.
The default value NO_PRIORITY means that the priority for statements is not forced to change.

Note

Starting from v6.6.0, TiDB supports Resource Control. You can use this feature to execute SQL statements with different priorities in different resource groups. By configuring proper quotas and priorities for these resource groups, you can gain better scheduling control for SQL statements with different priorities. When resource control is enabled, statement priority will no longer take effect. It is recommended that you use Resource Control to manage resource usage for different SQL statements.

tidb_gc_concurrency New in v5.0

Note

This variable is read-only for TiDB Serverless.

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: -1
Range: -1 or [1, 256]
Unit: Threads
This variable controls the number of concurrent threads during the Resolve Locks step of the Garbage Collection (GC) process.
Starting from v8.3.0, this variable also controls the number of concurrent threads during the Delete Ranges step of the GC process.
By default, this variable is -1, allowing TiDB to automatically determine the appropriate number of threads based on workloads.
When this variable is set to a number in the range of [1, 256]:
- Resolve Locks directly uses the value set for this variable as the number of threads.
- Delete Range uses one-fourth of the value set for this variable as the number of threads.

tidb_gc_enable New in v5.0

Note

This variable is read-only for TiDB Serverless.

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: ON
Enables garbage collection for TiKV. Disabling garbage collection will reduce system performance, as old versions of rows will no longer be purged.

tidb_gc_life_time New in v5.0

Note

This variable is read-only for TiDB Serverless.

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Duration
Default value: 10m0s
Range: [10m0s, 8760h0m0s]
The time limit during which data is retained for each GC, in the format of Go Duration. When a GC happens, the current time minus this value is the safe point.

Note

In scenarios of frequent updates, a large value (days or even months) for tidb_gc_life_time may cause potential issues, such as:
- Larger storage use
- A large amount of history data may affect performance to a certain degree, especially for range queries such as select count(*) from t
If there is any transaction that has been running longer than tidb_gc_life_time, during GC, the data since start_ts is retained for this transaction to continue execution. For example, if tidb_gc_life_time is configured to 10 minutes, among all transactions being executed, the transaction that starts earliest has been running for 15 minutes, GC will retain data of the recent 15 minutes.

tidb_gc_max_wait_time New in v6.1.0

Note

This variable is read-only for TiDB Serverless.

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 86400
Range: [600, 31536000]
Unit: Seconds
This variable is used to set the maximum time that active transactions block the GC safe point. During each time of GC, the safe point does not exceed the start time of the ongoing transactions by default. If the runtime of active transactions does not exceed this variable value, the GC safe point will be blocked until the runtime exceeds this value.

tidb_gc_run_interval New in v5.0

Note

This variable is read-only for TiDB Serverless.

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Duration
Default value: 10m0s
Range: [10m0s, 8760h0m0s]
Specifies the GC interval, in the format of Go Duration, for example, "1h30m", and "15m"

tidb_gc_scan_lock_mode New in v5.0

Warning

Currently, Green GC is an experimental feature. It is not recommended that you use it in production environments.

Note

This variable is read-only for TiDB Serverless.

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Enumeration
Default value: LEGACY
Possible values: PHYSICAL, LEGACY
- LEGACY: Uses the old way of scanning, that is, disable Green GC.
- PHYSICAL: Uses the physical scanning method, that is, enable Green GC.

This variable specifies the way of scanning locks in the Resolve Locks step of GC. When the variable value is set to LEGACY, TiDB scans locks by Regions. When the value PHYSICAL is used, it enables each TiKV node to bypass the Raft layer and directly scan data, which can effectively mitigate the impact of GC wakening up all Regions when the Hibernate Region feature is enabled, thus improving the execution speed in the Resolve Locks step.

tidb_general_log

Note

This TiDB variable is not applicable to TiDB Cloud.

Scope: GLOBAL
Persists to cluster: No, only applicable to the current TiDB instance that you are connecting to.
Applies to hint SET_VAR: No
Type: Boolean
Default value: OFF

This variable is used to set whether to record all SQL statements in the log. This feature is disabled by default. If maintenance personnel needs to trace all SQL statements when locating issues, they can enable this feature.
To see all records of this feature in the log, you need to set the TiDB configuration item log.level to "info" or "debug" and then query the "GENERAL_LOG" string. The following information is recorded:
- conn: The ID of the current session.
- user: The current session user.
- schemaVersion: The current schema version.
- txnStartTS: The timestamp at which the current transaction starts.
- forUpdateTS: In the pessimistic transactional mode, forUpdateTS is the current timestamp of the SQL statement. When a write conflict occurs in the pessimistic transaction, TiDB retries the SQL statement currently being executed and updates this timestamp. You can configure the number of retries via max-retry-count. In the optimistic transactional model, forUpdateTS is equivalent to txnStartTS.
- isReadConsistency: Indicates whether the current transactional isolation level is Read Committed (RC).
- current_db: The name of the current database.
- txn_mode: The transactional mode. Value options are OPTIMISTIC and PESSIMISTIC.
- sql: The SQL statement corresponding to the current query.

tidb_non_prepared_plan_cache_size

Warning

Starting from v7.1.0, this variable is deprecated. Instead, use tidb_session_plan_cache_size for setting.

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 100
Range: [1, 100000]
This variable controls the maximum number of execution plans that can be cached by Non-prepared plan cache.

tidb_generate_binary_plan New in v6.2.0

Note

This variable is read-only for TiDB Serverless.

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: ON
This variable controls whether to generate binary-encoded execution plans in slow logs and statement summaries.
When this variable is set to ON, you can view visual execution plans in TiDB Dashboard. Note that TiDB Dashboard only provides visual display for execution plans generated after this variable is enabled.
You can execute the SELECT tidb_decode_binary_plan('xxx...') statement to parse the specific plan from a binary plan.

tidb_gogc_tuner_max_value New in v7.5.0

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 500
Range: [10, 2147483647]
The variable is used to control the maximum value of GOGC that the GOGC Tuner can adjust.

tidb_gogc_tuner_min_value New in v7.5.0

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 100
Range: [10, 2147483647]
The variable is used to control the minimum value of GOGC that the GOGC Tuner can adjust.

tidb_gogc_tuner_threshold New in v6.4.0

Note

This variable is read-only for TiDB Serverless.

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Default value: 0.6
Range: [0, 0.9)
This variable specifies the maximum memory threshold for tuning GOGC. When the memory exceeds this threshold, GOGC Tuner stops working.

tidb_guarantee_linearizability New in v5.0

Note

This variable is read-only for TiDB Serverless.

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: ON
This variable controls the way commit TS is calculated for async commit. By default (with the ON value), the two-phase commit requests a new TS from the PD server and uses the TS to calculate the final commit TS. In this situation, linearizability is guaranteed for all the concurrent transactions.
If you set this variable to OFF, the process of fetching TS from the PD server is skipped, with the cost that only causal consistency is guaranteed but not linearizability. For more details, see the blog post Async Commit, the Accelerator for Transaction Commit in TiDB 5.0.
For scenarios that require only causal consistency, you can set this variable to OFF to improve performance.

tidb_hash_exchange_with_new_collation

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: ON
This variable controls whether the MPP hash partition exchange operator is generated in a cluster with new collation enabled. true means to generate the operator, and false means not to generate it.
This variable is used for the internal operation of TiDB. It is NOT recommended to set this variable.

tidb_hash_join_concurrency

Warning

Since v5.0, this variable is deprecated. Instead, use tidb_executor_concurrency for setting.

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: -1
Range: [1, 256]
Unit: Threads
This variable is used to set the concurrency of the hash join algorithm.
A value of -1 means that the value of tidb_executor_concurrency will be used instead.

tidb_hashagg_final_concurrency

Warning

Since v5.0, this variable is deprecated. Instead, use tidb_executor_concurrency for setting.

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: -1
Range: [1, 256]
Unit: Threads
This variable is used to set the concurrency of executing the concurrent hash aggregation algorithm in the final phase.
When the parameter of the aggregate function is not distinct, HashAgg is run concurrently and respectively in two phases - the partial phase and the final phase.
A value of -1 means that the value of tidb_executor_concurrency will be used instead.

tidb_hashagg_partial_concurrency

Warning

Since v5.0, this variable is deprecated. Instead, use tidb_executor_concurrency for setting.

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: -1
Range: [1, 256]
Unit: Threads
This variable is used to set the concurrency of executing the concurrent hash aggregation algorithm in the partial phase.
When the parameter of the aggregate function is not distinct, HashAgg is run concurrently and respectively in two phases - the partial phase and the final phase.
A value of -1 means that the value of tidb_executor_concurrency will be used instead.

tidb_historical_stats_duration New in v6.6.0

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Duration
Default value: 168h, which means 7 days
This variable controls the duration that the historical statistics are retained in the storage.

tidb_idle_transaction_timeout New in v7.6.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 0
Range: [0, 31536000]
Unit: Seconds
This variable controls the idle timeout for transactions in a user session. When a user session is in a transactional state and remains idle for a duration exceeding the value of this variable, TiDB will terminate the session. An idle user session means that there are no active requests and the session is waiting for new requests.
The default value 0 means unlimited.

tidb_ignore_prepared_cache_close_stmt New in v6.0.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: OFF
This variable is used to set whether to ignore the commands for closing prepared statement cache.
When this variable is set to ON, the COM_STMT_CLOSE command of the Binary protocol and the DEALLOCATE PREPARE statement of the text protocol are ignored. For details, see Ignore the COM_STMT_CLOSE command and the DEALLOCATE PREPARE statement.

tidb_index_join_batch_size

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Integer
Default value: 25000
Range: [1, 2147483647]
Unit: Rows
This variable is used to set the batch size of the index lookup join operation.
Use a bigger value in OLAP scenarios, and a smaller value in OLTP scenarios.

tidb_index_join_double_read_penalty_cost_rate New in v6.6.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Float
Default value: 0
Range: [0, 18446744073709551615]
This variable determines whether to apply a penalty cost to the selection of index join, which reduces the likelihood of the optimizer selecting index join, and increases the likelihood of selecting alternative join methods such as hash join and tiflash join.
When index join is selected, many table lookup requests are triggered, which consumes too many resources. You can use this variable to reduce the likelihood of the optimizer selecting index join.
This variable takes effect only when the tidb_cost_model_version variable is set to 2.

tidb_index_lookup_concurrency

Warning

Since v5.0, this variable is deprecated. Instead, use tidb_executor_concurrency for setting.

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: -1
Range: [1, 256]
Unit: Threads
This variable is used to set the concurrency of the index lookup operation.
Use a bigger value in OLAP scenarios, and a smaller value in OLTP scenarios.
A value of -1 means that the value of tidb_executor_concurrency will be used instead.

tidb_index_lookup_join_concurrency

Warning

Since v5.0, this variable is deprecated. Instead, use tidb_executor_concurrency for setting.

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: -1
Range: [1, 256]
Unit: Threads
This variable is used to set the concurrency of the index lookup join algorithm.
A value of -1 means that the value of tidb_executor_concurrency will be used instead.

tidb_index_merge_intersection_concurrency New in v6.5.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Default value: -1
Range: [1, 256]
This variable sets the maximum concurrency for the intersection operations that index merge performs. It is effective only when TiDB accesses partitioned tables in the dynamic pruning mode. The actual concurrency is the smaller value of tidb_index_merge_intersection_concurrency and the number of partitions of the partitioned table.
The default value -1 means that the value of tidb_executor_concurrency is used.

tidb_index_lookup_size

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Integer
Default value: 20000
Range: [1, 2147483647]
Unit: Rows
This variable is used to set the batch size of the index lookup operation.
Use a bigger value in OLAP scenarios, and a smaller value in OLTP scenarios.

tidb_index_serial_scan_concurrency

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Integer
Default value: 1
Range: [1, 256]
Unit: Threads
This variable is used to set the concurrency of the serial scan operation.
Use a bigger value in OLAP scenarios, and a smaller value in OLTP scenarios.

tidb_init_chunk_size

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 32
Range: [1, 32]
Unit: Rows
This variable is used to set the number of rows for the initial chunk during the execution process. The number of rows for a chunk directly affects the amount of memory required for a single query. You can roughly estimate the memory needed for a single chunk by considering the total width of all columns in the query and the number of rows for the chunk. Combining this with the concurrency of the executor, you can make a rough estimation of the total memory required for a single query. It is recommended that the total memory for a single chunk does not exceed 16 MiB.

tidb_isolation_read_engines New in v4.0

Note

This variable is read-only for TiDB Serverless.

Scope: SESSION
Applies to hint SET_VAR: Yes
Default value: tikv,tiflash,tidb
This variable is used to set the storage engine list that TiDB can use when reading data.

tidb_last_ddl_info New in v6.0.0

Scope: SESSION
Applies to hint SET_VAR: No
Default value: ""
Type: String
This is a read-only variable. It is internally used in TiDB to get the information of the last DDL operation within the current session.
- "query": The last DDL query string.
- "seq_num": The sequence number for each DDL operation. It is used to identify the order of DDL operations.

tidb_last_query_info New in v4.0.14

Scope: SESSION
Applies to hint SET_VAR: No
Default value: ""
This is a read-only variable. It is internally used in TiDB to query the transaction information of the last DML statement. The information includes:
- txn_scope: The scope of the transaction, which can be global or local.
- start_ts: The start timestamp of the transaction.
- for_update_ts: The for_update_ts of the previously executed DML statement. This is an internal term of TiDB used for tests. Usually, you can ignore this information.
- error: The error message, if any.
- ru_consumption: Consumed RU for executing the statement.

tidb_last_txn_info New in v4.0.9

Scope: SESSION
Applies to hint SET_VAR: No
Type: String
This variable is used to get the last transaction information within the current session. It is a read-only variable. The transaction information includes:
- The transaction scope.
- The start and commit TS.
- The transaction commit mode, which might be a two-phase, one-phase, or async commit.
- The information of transaction fallback from async commit or one-phase commit to two-phase commit.
- The error encountered.

tidb_last_plan_replayer_token New in v6.3.0

Scope: SESSION
Applies to hint SET_VAR: No
Type: String
This variable is read-only and is used to obtain the result of the last PLAN REPLAYER DUMP execution in the current session.

tidb_load_based_replica_read_threshold New in v7.0.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Default value: "1s"
Range: [0s, 1h]
Type: String
This variable is used to set the threshold for triggering load-based replica read. When the estimated queue time of the leader node exceeds the threshold, TiDB prioritizes reading data from the follower node. The format is a time duration, such as "100ms" or "1s". For more details, see Troubleshoot Hotspot Issues.

`tidb_load_binding_timeout` New in v8.0.0

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 200
Range: (0, 2147483647]
Unit: Milliseconds
This variable is used to control the timeout of loading bindings. If the execution time of loading bindings exceeds this value, the loading will stop.

`tidb_lock_unchanged_keys` New in v7.1.1 and v7.3.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: ON
This variable is used to control whether to lock specific keys in the following scenarios. When the value is set to ON, these keys are locked. When the value is set to OFF, these keys are not locked.
- Duplicate keys in INSERT IGNORE and REPLACE statements. Before v6.1.6, these keys were not locked. This issue has been fixed in #42121.
- Unique keys in UPDATE statements when the values of the keys are not changed. Before v6.5.2, these keys were not locked. This issue has been fixed in #36438.
To maintain the consistency and rationality of the transaction, it is not recommended to change this value. If upgrading TiDB causes severe performance issues due to these two fixes, and the behavior without locks is acceptable (see the preceding issues), you can set this variable to OFF.

tidb_log_file_max_days New in v5.3.0

Note

This variable is read-only for TiDB Serverless.

Scope: GLOBAL
Persists to cluster: No, only applicable to the current TiDB instance that you are connecting to.
Applies to hint SET_VAR: No
Type: Integer
Default value: 0
Range: [0, 2147483647]

This variable is used to set the maximum days that the log is retained on the current TiDB instance. Its value defaults to the value of the max-days configuration in the configuration file. Changing the variable value only affects the current TiDB instance. After TiDB is restarted, the variable value is reset and the configuration value is not affected.

tidb_low_resolution_tso

Scope: SESSION | GLOBAL
Applies to hint SET_VAR: No
Type: Boolean
Default value: OFF
This variable is used to set whether to enable the low-precision TSO feature. After this feature is enabled, TiDB uses the cached timestamp to read data. The cached timestamp is updated every 2 seconds by default. Starting from v8.0.0, you can configure the update interval by tidb_low_resolution_tso_update_interval.
The main applicable scenario is to reduce the overhead of acquiring TSO for small read-only transactions when reading old data is acceptable.
Starting from v8.3.0, this variable supports the GLOBAL scope.

`tidb_low_resolution_tso_update_interval` New in v8.0.0

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 2000
Range: [10, 60000]
Unit: Milliseconds
This variable is used to set the update interval of the cached timestamp used in the low-precision TSO feature, in milliseconds.
This variable is only available when tidb_low_resolution_tso is enabled.

tidb_max_auto_analyze_time New in v6.1.0

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 43200
Range: [0, 2147483647]
Unit: Seconds
This variable is used to specify the maximum execution time of automatic ANALYZE tasks. When the execution time of an automatic ANALYZE task exceeds the specified time, the task will be terminated. When the value of this variable is 0, there is no limit to the maximum execution time of automatic ANALYZE tasks.

tidb_max_bytes_before_tiflash_external_group_by New in v7.0.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Integer
Default value: -1
Range: [-1, 9223372036854775807]
This variable is used to specify the maximum memory usage of the Hash Aggregation operator with GROUP BY in TiFlash, in bytes. When the memory usage exceeds the specified value, TiFlash triggers the Hash Aggregation operator to spill to disk. When the value of this variable is -1, TiDB does not pass this variable to TiFlash. Only when the value of this variable is greater than or equal to 0, TiDB passes this variable to TiFlash. When the value of this variable is 0, it means that the memory usage is unlimited, that is, TiFlash Hash Aggregation operator will not trigger spilling. For details, see TiFlash Spill to Disk.

Note

If a TiDB cluster has multiple TiFlash nodes, aggregation is usually distributedly executed on multiple TiFlash nodes. This variable controls the maximum memory usage of the aggregation operator on a single TiFlash node.
When this variable is set to -1, TiFlash determines the maximum memory usage of the aggregation operator based on the value of its own configuration item max_bytes_before_external_group_by.

tidb_max_bytes_before_tiflash_external_join New in v7.0.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Integer
Default value: -1
Range: [-1, 9223372036854775807]
This variable is used to specify the maximum memory usage of the Hash Join operator with JOIN in TiFlash, in bytes. When the memory usage exceeds the specified value, TiFlash triggers the Hash Join operator to spill to disk. When the value of this variable is -1, TiDB does not pass this variable to TiFlash. Only when the value of this variable is greater than or equal to 0, TiDB passes this variable to TiFlash. When the value of this variable is 0, it means that the memory usage is unlimited, that is, TiFlash Hash Join operator will not trigger spilling. For details, see TiFlash Spill to Disk.

Note

If a TiDB cluster has multiple TiFlash nodes, join is usually distributedly executed on multiple TiFlash nodes. This variable controls the maximum memory usage of the join operator on a single TiFlash node.
When this variable is set to -1, TiFlash determines the maximum memory usage of the join operator based on the value of its own configuration item max_bytes_before_external_join.

tidb_max_bytes_before_tiflash_external_sort New in v7.0.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Integer
Default value: -1
Range: [-1, 9223372036854775807]
This variable is used to specify the maximum memory usage of the TopN and Sort operators in TiFlash, in bytes. When the memory usage exceeds the specified value, TiFlash triggers the TopN and Sort operators to spill to disk. When the value of this variable is -1, TiDB does not pass this variable to TiFlash. Only when the value of this variable is greater than or equal to 0, TiDB passes this variable to TiFlash. When the value of this variable is 0, it means that the memory usage is unlimited, that is, TiFlash TopN and Sort operators will not trigger spilling. For details, see TiFlash Spill to Disk.

Note

If a TiDB cluster has multiple TiFlash nodes, TopN and Sort are usually distributedly executed on multiple TiFlash nodes. This variable controls the maximum memory usage of the TopN and Sort operators on a single TiFlash node.
When this variable is set to -1, TiFlash determines the maximum memory usage of the TopN and Sort operators based on the value of its own configuration item max_bytes_before_external_sort.

tidb_max_chunk_size

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 1024
Range: [32, 2147483647]
Unit: Rows
This variable is used to set the maximum number of rows in a chunk during the execution process. Setting to too large of a value may cause cache locality issues. The recommended value for this variable is no larger than 65536. The number of rows for a chunk directly affects the amount of memory required for a single query. You can roughly estimate the memory needed for a single chunk by considering the total width of all columns in the query and the number of rows for the chunk. Combining this with the concurrency of the executor, you can make a rough estimation of the total memory required for a single query. It is recommended that the total memory for a single chunk does not exceed 16 MiB. When the query involves a large amount of data and a single chunk is insufficient to handle all the data, TiDB processes it multiple times, doubling the chunk size with each processing iteration, starting from tidb_init_chunk_size until the chunk size reaches the value of tidb_max_chunk_size.

tidb_max_delta_schema_count New in v2.1.18 and v3.0.5

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 1024
Range: [100, 16384]
This variable is used to set the maximum number of schema versions (the table IDs modified for corresponding versions) allowed to be cached. The value range is 100 ~ 16384.

tidb_max_paging_size New in v6.3.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Integer
Default value: 50000
Range: [1, 9223372036854775807]
Unit: Rows
This variable is used to set the maximum number of rows during the coprocessor paging request process. Setting it to too small a value increases the RPC count between TiDB and TiKV, while setting it to too large a value results in excessive memory usage in some cases, such as loading data and full table scan. The default value of this variable brings better performance in OLTP scenarios than in OLAP scenarios. If the application only uses TiKV as the storage engine, consider increasing the value of this variable when executing OLAP workload queries, which might bring you better performance.

tidb_max_tiflash_threads New in v6.1.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: -1
Range: [-1, 256]
Unit: Threads
This variable is used to set the maximum concurrency for TiFlash to execute a request. The default value is -1, indicating that this system variable is invalid and the maximum concurrency depends on the setting of the TiFlash configuration profiles.default.max_threads. When the value is 0, the maximum number of threads is automatically configured by TiFlash.

tidb_mem_oom_action New in v6.1.0

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Enumeration
Default value: CANCEL
Possible values: CANCEL, LOG

Specifies what operation TiDB performs when a single SQL statement exceeds the memory quota specified by tidb_mem_quota_query and cannot be spilled over to disk. See TiDB Memory Control for details.

The default value is CANCEL, but in TiDB v4.0.2 and earlier versions, the default value is LOG.
This setting was previously a tidb.toml option (oom-action), but changed to a system variable starting from TiDB v6.1.0.

tidb_mem_quota_analyze New in v6.1.0

Warning

Currently, the ANALYZE memory quota is an experimental feature, and the memory statistics might be inaccurate in production environments.

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: -1
Range: [-1, 9223372036854775807]
Unit: Bytes
This variable controls the maximum memory usage of TiDB updating statistics. Such a memory usage occurs when you manually execute ANALYZE TABLE and when TiDB automatically analyzes tasks in the background. When the total memory usage exceeds this threshold, user-executed ANALYZE will exit, and an error message is reported that reminds you to try a lower sampling rate or retry later. If the automatic task in the TiDB background exits because the memory threshold is exceeded, and the sampling rate used is higher than the default value, TiDB will retry the update using the default sampling rate. When this variable value is negative or zero, TiDB does not limit the memory usage of both the manual and automatic update tasks.

Note

auto_analyze will be triggered in a TiDB cluster only when run-auto-analyze is enabled in the TiDB startup configuration file.

tidb_mem_quota_apply_cache New in v5.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 33554432 (32 MiB)
Range: [0, 9223372036854775807]
Unit: Bytes
This variable is used to set the memory usage threshold of the local cache in the Apply operator.
The local cache in the Apply operator is used to speed up the computation of the Apply operator. You can set the variable to 0 to disable the Apply cache feature.

tidb_mem_quota_binding_cache New in v6.0.0

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 67108864
Range: [0, 2147483647]
Unit: Bytes
This variable is used to set the threshold of the memory used for caching bindings.
If a system creates or captures excessive bindings, resulting in overuse of memory space, TiDB returns a warning in the log. In this case, the cache cannot hold all available bindings or determine which bindings to store. For this reason, some queries might miss their bindings. To address this problem, you can increase the value of this variable, which increases the memory used for caching bindings. After modifying this parameter, you need to run admin reload bindings to reload bindings and validate the modification.

tidb_mem_quota_query

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 1073741824 (1 GiB)
Range: [-1, 9223372036854775807]
Unit: Bytes

For versions earlier than TiDB v6.1.0, this is a session scope variable and uses the value of mem-quota-query from tidb.toml as an initial value. Starting from v6.1.0, tidb_mem_quota_query is a SESSION | GLOBAL scope variable.
For versions earlier than TiDB v6.5.0, this variable is used to set the threshold value of memory quota for a query. If the memory quota of a query during execution exceeds the threshold value, TiDB performs the operation defined by tidb_mem_oom_action.
For TiDB v6.5.0 and later versions, this variable is used to set the threshold value of memory quota for a session. If the memory quota of a session during execution exceeds the threshold value, TiDB performs the operation defined by tidb_mem_oom_action. Note that starting from TiDB v6.5.0, the memory usage of a session contains the memory consumed by the transactions in the session. For the control behavior of transaction memory usage in TiDB v6.5.0 and later versions, see txn-total-size-limit.
When you set the variable value to 0 or -1, the memory threshold is positive infinity. When you set a value smaller than 128, the value will be defaulted to 128.

tidb_memory_debug_mode_alarm_ratio

Scope: SESSION
Applies to hint SET_VAR: No
Type: Float
Default value: 0
This variable represents the memory statistics error value allowed in the TiDB memory debug mode.
This variable is used for the internal testing of TiDB. It is NOT recommended to set this variable.

tidb_memory_debug_mode_min_heap_inuse

Scope: SESSION
Applies to hint SET_VAR: No
Type: Integer
Default value: 0
This variable is used for the internal testing of TiDB. It is NOT recommended to set this variable. Enabling this variable will affect the performance of TiDB.
After configuring this parameter, TiDB will enter the memory debug mode to analyze the accuracy of memory tracking. TiDB will frequently trigger GC during the execution of subsequent SQL statements, and compare the actual memory usage and memory statistics. If the current memory usage is greater than tidb_memory_debug_mode_min_heap_inuse and the memory statistics error exceeds tidb_memory_debug_mode_alarm_ratio, TiDB will output the relevant memory information to the log and file.

tidb_memory_usage_alarm_ratio

Note

This TiDB variable is not applicable to TiDB Cloud.

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Float
Default value: 0.7
Range: [0.0, 1.0]

This variable sets the memory usage ratio that triggers the tidb-server memory alarm. By default, TiDB prints an alarm log when TiDB memory usage exceeds 70% of its total memory and any of the alarm conditions is met.
When this variable is configured to 0 or 1, it means the memory threshold alarm feature is disabled.
When this variable is configured to a value greater than 0 and less than 1, it means that the memory threshold alarm feature is enabled.
- If the value of the system variable tidb_server_memory_limit is 0, the memory alarm threshold is tidb_memory-usage-alarm-ratio * system memory size.
- If the value of the system variable tidb_server_memory_limit is set to greater than 0, the memory alarm threshold is tidb_memory-usage-alarm-ratio * tidb_server_memory_limit.

tidb_memory_usage_alarm_keep_record_num New in v6.4.0

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Default value: 5
Range: [1, 10000]
When the tidb-server memory usage exceeds the memory alarm threshold and triggers an alarm, TiDB only retains the status files generated during the recent 5 alarms by default. You can adjust this number with this variable.

tidb_merge_join_concurrency

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Integer
Range: [1, 256]
Default value: 1
This variable sets the concurrency of the MergeJoin operator when a query is executed.
It is NOT recommended to set this variable. Modifying the value of this variable might cause data correctness issues.

tidb_merge_partition_stats_concurrency

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Default value: 1
This variable specifies the concurrency of merging statistics for a partitioned table when TiDB analyzes the partitioned table.

tidb_enable_async_merge_global_stats New in v7.5.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: ON. When you upgrade TiDB from a version earlier than v7.5.0 to v7.5.0 or a later version, the default value is OFF.
This variable is used for TiDB to merge global statistics asynchronously to avoid OOM issues.

tidb_metric_query_range_duration New in v4.0

Note

This TiDB variable is not applicable to TiDB Cloud.

Scope: SESSION
Applies to hint SET_VAR: No
Type: Integer
Default value: 60
Range: [10, 216000]
Unit: Seconds
This variable is used to set the range duration of the Prometheus statement generated when querying METRICS_SCHEMA.

tidb_metric_query_step New in v4.0

Note

This TiDB variable is not applicable to TiDB Cloud.

Scope: SESSION
Applies to hint SET_VAR: No
Type: Integer
Default value: 60
Range: [10, 216000]
Unit: Seconds
This variable is used to set the step of the Prometheus statement generated when querying METRICS_SCHEMA.

tidb_min_paging_size New in v6.2.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Integer
Default value: 128
Range: [1, 9223372036854775807]
Unit: Rows
This variable is used to set the minimum number of rows during the coprocessor paging request process. Setting it to a too small value increases the RPC request count between TiDB and TiKV, while setting it to a too large value might cause a performance decrease when executing queries using IndexLookup with Limit. The default value of this variable brings better performance in OLTP scenarios than in OLAP scenarios. If the application only uses TiKV as the storage engine, consider increasing the value of this variable when executing OLAP workload queries, which might bring you better performance.

Paging size impact on TPCH

As shown in this diagram, when tidb_enable_paging is enabled, the performance of TPCH is affected by the settings of tidb_min_paging_size and tidb_max_paging_size. The vertical axis is the execution time, and it is the smaller the better.

tidb_mpp_store_fail_ttl

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Duration
Default value: 60s
The newly started TiFlash node does not provide services. To prevent queries from failing, TiDB limits the tidb-server sending queries to the newly started TiFlash node. This variable indicates the time range in which the newly started TiFlash node is not sent requests.

tidb_multi_statement_mode New in v4.0.11

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Enumeration
Default value: OFF
Possible values: OFF, ON, WARN
This variable controls whether to allow multiple queries to be executed in the same COM_QUERY call.
To reduce the impact of SQL injection attacks, TiDB now prevents multiple queries from being executed in the same COM_QUERY call by default. This variable is intended to be used as part of an upgrade path from earlier versions of TiDB. The following behaviors apply:

Client setting	`tidb_multi_statement_mode` value	Multiple statements permitted?
Multiple Statements = ON	OFF	Yes
Multiple Statements = ON	ON	Yes
Multiple Statements = ON	WARN	Yes
Multiple Statements = OFF	OFF	No
Multiple Statements = OFF	ON	Yes
Multiple Statements = OFF	WARN	Yes (+warning returned)

Note

Only the default value of OFF can be considered safe. Setting tidb_multi_statement_mode=ON might be required if your application was specifically designed for an earlier version of TiDB. If your application requires multiple statement support, it is recommended to use the setting provided by your client library instead of the tidb_multi_statement_mode option. For example:

go-sql-driver (multiStatements)
Connector/J (allowMultiQueries)
PHP mysqli (mysqli_multi_query)

tidb_nontransactional_ignore_error New in v6.1.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: OFF
This variable specifies whether to return an error immediately when the error occurs in a non-transactional DML statement.
When the value is set to OFF, the non-transactional DML statement stops immediately at the first error and returns the error. All the following batches are canceled.
When the value is set to ON and an error occurs in a batch, the following batches will continue to be executed until all batches are executed. All errors occurred during the execution process are returned together in the result.

tidb_opt_agg_push_down

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Boolean
Default value: OFF
This variable is used to set whether the optimizer executes the optimization operation of pushing down the aggregate function to the position before Join, Projection, and UnionAll.
When the aggregate operation is slow in query, you can set the variable value to ON.

tidb_opt_broadcast_cartesian_join

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Integer
Default value: 1
Range: [0, 2]
Indicates whether to allow the Broadcast Cartesian Join.
0 means that the Broadcast Cartesian Join is not allowed. 1 means that it is allowed based on tidb_broadcast_join_threshold_count. 2 means that it is always allowed even if the table size exceeds the threshold.
This variable is internally used in TiDB, and it is NOT recommended to modify its value.

tidb_opt_concurrency_factor

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Float
Range: [0, 18446744073709551615]
Default value: 3.0
Indicates the CPU cost of starting a Golang goroutine in TiDB. This variable is internally used in the Cost Model, and it is NOT recommended to modify its value.

tidb_opt_copcpu_factor

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Float
Range: [0, 18446744073709551615]
Default value: 3.0
Indicates the CPU cost for TiKV Coprocessor to process one row. This variable is internally used in the Cost Model, and it is NOT recommended to modify its value.

tidb_opt_correlation_exp_factor

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Integer
Default value: 1
Range: [0, 2147483647]
When the method that estimates the number of rows based on column order correlation is not available, the heuristic estimation method is used. This variable is used to control the behavior of the heuristic method.
- When the value is 0, the heuristic method is not used.
- When the value is greater than 0:
  - A larger value indicates that an index scan will probably be used in the heuristic method.
  - A smaller value indicates that a table scan will probably be used in the heuristic method.

tidb_opt_correlation_threshold

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Float
Default value: 0.9
Range: [0, 1]
This variable is used to set the threshold value that determines whether to enable estimating the row count by using column order correlation. If the order correlation between the current column and the handle column exceeds the threshold value, this method is enabled.

tidb_opt_cpu_factor

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Float
Range: [0, 2147483647]
Default value: 3.0
Indicates the CPU cost for TiDB to process one row. This variable is internally used in the Cost Model, and it is NOT recommended to modify its value.

`tidb_opt_derive_topn` New in v7.0.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Boolean
Default value: OFF
Controls whether to enable the optimization rule of Deriving TopN or Limit from window functions.

tidb_opt_desc_factor

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Float
Range: [0, 18446744073709551615]
Default value: 3.0
Indicates the cost for TiKV to scan one row from the disk in descending order. This variable is internally used in the Cost Model, and it is NOT recommended to modify its value.

tidb_opt_disk_factor

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Float
Range: [0, 18446744073709551615]
Default value: 1.5
Indicates the I/O cost for TiDB to read or write one byte of data from or to the temporary disk. This variable is internally used in the Cost Model, and it is NOT recommended to modify its value.

tidb_opt_distinct_agg_push_down

Scope: SESSION
Applies to hint SET_VAR: Yes
Type: Boolean
Default value: OFF
This variable is used to set whether the optimizer executes the optimization operation of pushing down the aggregate function with distinct (such as select count(distinct a) from t) to Coprocessor.
When the aggregate function with the distinct operation is slow in the query, you can set the variable value to 1.

In the following example, before tidb_opt_distinct_agg_push_down is enabled, TiDB needs to read all data from TiKV and execute distinct on the TiDB side. After tidb_opt_distinct_agg_push_down is enabled, distinct a is pushed down to Coprocessor, and a group by column test.t.a is added to HashAgg_5.

mysql> desc select count(distinct a) from test.t;
+-------------------------+----------+-----------+---------------+------------------------------------------+
| id                      | estRows  | task      | access object | operator info                            |
+-------------------------+----------+-----------+---------------+------------------------------------------+
| StreamAgg_6             | 1.00     | root      |               | funcs:count(distinct test.t.a)->Column#4 |
| └─TableReader_10        | 10000.00 | root      |               | data:TableFullScan_9                     |
|   └─TableFullScan_9     | 10000.00 | cop[tikv] | table:t       | keep order:false, stats:pseudo           |
+-------------------------+----------+-----------+---------------+------------------------------------------+
3 rows in set (0.01 sec)

mysql> set session tidb_opt_distinct_agg_push_down = 1;
Query OK, 0 rows affected (0.00 sec)

mysql> desc select count(distinct a) from test.t;
+---------------------------+----------+-----------+---------------+------------------------------------------+
| id                        | estRows  | task      | access object | operator info                            |
+---------------------------+----------+-----------+---------------+------------------------------------------+
| HashAgg_8                 | 1.00     | root      |               | funcs:count(distinct test.t.a)->Column#3 |
| └─TableReader_9           | 1.00     | root      |               | data:HashAgg_5                           |
|   └─HashAgg_5             | 1.00     | cop[tikv] |               | group by:test.t.a,                       |
|     └─TableFullScan_7     | 10000.00 | cop[tikv] | table:t       | keep order:false, stats:pseudo           |
+---------------------------+----------+-----------+---------------+------------------------------------------+
4 rows in set (0.00 sec)

tidb_opt_enable_correlation_adjustment

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Boolean
Default value: ON
This variable is used to control whether the optimizer estimates the number of rows based on column order correlation

tidb_opt_enable_hash_join New in v6.5.6, v7.1.2, and v7.4.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: ON
This variable is used to control whether the optimizer selects hash joins for tables. The value is ON by default. If it is set to OFF, the optimizer avoids selecting hash joins when generating execution plans, unless no other join algorithm is available.
If both the system variable tidb_opt_enable_hash_join and the HASH_JOIN hint are configured, the HASH_JOIN hint takes precedence. Even if tidb_opt_enable_hash_join is set to OFF, when you specify a HASH_JOIN hint in a query, the TiDB optimizer still enforces a hash join plan.

tidb_opt_enable_non_eval_scalar_subquery New in v7.3.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: OFF
This variable is used to control whether the EXPLAIN statement disables the execution of constant subqueries that can be expanded at the optimization stage. When this variable is set to OFF, the EXPLAIN statement expands the subquery in advance at the optimization stage. When this variable is set to ON, the EXPLAIN statement does not expand the subquery at the optimization stage. For more information, see Disable subquery expansion.

tidb_opt_enable_late_materialization New in v7.0.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Boolean
Default value: ON
This variable is used to control whether to enable the TiFlash late materialization feature. Note that TiFlash late materialization does not take effect in the fast scan mode.
When this variable is set to OFF to disable the TiFlash late materialization feature, to process a SELECT statement with filter conditions (WHERE clause), TiFlash scans all the data of the required columns before filtering. When this variable is set to ON to enable the TiFlash late materialization feature, TiFlash can first scan the column data related to the filter conditions that are pushed down to the TableScan operator, filter the rows that meet the conditions, and then scan the data of other columns of these rows for further calculations, thereby reducing IO scans and computations of data processing.

tidb_opt_enable_mpp_shared_cte_execution New in v7.2.0

Warning

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Boolean
Default value: OFF
This variable controls whether the non-recursive Common Table Expressions (CTE) can be executed on TiFlash MPP. By default, when this variable is disabled, CTE is executed on TiDB, which has a large performance gap compared with enabling this feature.

tidb_opt_enable_fuzzy_binding New in v7.6.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Boolean
Default value: OFF
This variable controls whether to enable the Cross-database binding feature.

tidb_opt_fix_control New in v6.5.3 and v7.1.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: String
Default value: ""
This variable is used to control some internal behaviors of the optimizer.
The optimizer's behavior might vary depending on user scenarios or SQL statements. This variable provides a more fine-grained control over the optimizer and helps to prevent performance regression after upgrading caused by behavior changes in the optimizer.
For a more detailed introduction, see Optimizer Fix Controls.

tidb_opt_force_inline_cte New in v6.3.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Boolean
Default value: OFF
This variable is used to control whether common table expressions (CTEs) in the entire session are inlined or not. The default value is OFF, which means that inlining CTE is not enforced by default. However, you can still inline CTE by specifying the MERGE() hint. If the variable is set to ON, all CTEs (except recursive CTE) in this session are forced to be inlined.

tidb_opt_advanced_join_hint New in v7.0.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Boolean
Default value: ON
This variable is used to control whether the Join Method hint, such as HASH_JOIN() hint and MERGE_JOIN() hint, affects the Join Reorder optimization process, including the use of LEADING() hint. The default value is ON, which means that it does not affect. If it is set to OFF, there might be conflicts in some scenarios where both Join Method hint and LEADING() hint are used at the same time.

Note

The behavior of versions earlier than v7.0.0 is consistent with that of setting this variable to OFF. To ensure forward compatibility, when you upgrade from an earlier version to a v7.0.0 or later cluster, this variable is set to OFF. To obtain more flexible hint behavior, it is strongly recommended to switch this variable to ON under the condition that there is no performance regression.

tidb_opt_insubq_to_join_and_agg

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Boolean
Default value: ON
This variable is used to set whether to enable the optimization rule that converts a subquery to join and aggregation.
For example, after you enable this optimization rule, the subquery is converted as follows:
```
select * from t where t.a in (select aa from t1);
```
The subquery is converted to join as follows:
```
select t.* from t, (select aa from t1 group by aa) tmp_t where t.a = tmp_t.aa;
```
If t1 is limited to be unique and not null in the aa column. You can use the following statement, without aggregation.
```
select t.* from t, t1 where t.a=t1.aa;
```

tidb_opt_join_reorder_threshold

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Integer
Default value: 0
Range: [0, 2147483647]
This variable is used to control the selection of the TiDB Join Reorder algorithm. When the number of nodes participating in Join Reorder is greater than this threshold, TiDB selects the greedy algorithm, and when it is less than this threshold, TiDB selects the dynamic programming algorithm.
Currently, for OLTP queries, it is recommended to keep the default value. For OLAP queries, it is recommended to set the variable value to 10~15 to get better connection orders in OLAP scenarios.

tidb_opt_limit_push_down_threshold

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Integer
Default value: 100
Range: [0, 2147483647]
This variable is used to set the threshold that determines whether to push the Limit or TopN operator down to TiKV.
If the value of the Limit or TopN operator is smaller than or equal to this threshold, these operators are forcibly pushed down to TiKV. This variable resolves the issue that the Limit or TopN operator cannot be pushed down to TiKV partly due to wrong estimation.

tidb_opt_memory_factor

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Float
Range: [0, 2147483647]
Default value: 0.001
Indicates the memory cost for TiDB to store one row. This variable is internally used in the Cost Model, and it is NOT recommended to modify its value.

tidb_opt_mpp_outer_join_fixed_build_side New in v5.1.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Boolean
Default value: OFF
When the variable value is ON, the left join operator always uses inner table as the build side and the right join operator always uses outer table as the build side. If you set the value to OFF, the outer join operator can use either side of the tables as the build side.

tidb_opt_network_factor

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Float
Range: [0, 2147483647]
Default value: 1.0
Indicates the net cost of transferring 1 byte of data through the network. This variable is internally used in the Cost Model, and it is NOT recommended to modify its value.

tidb_opt_objective New in v7.4.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Enumeration
Default value: moderate
Possible values: moderate, determinate
This variable controls the objective of the optimizer. moderate maintains the default behavior in versions prior to TiDB v7.4.0, where the optimizer tries to use more information to generate better execution plans. determinate mode tends to be more conservative and makes the execution plan more stable.
The real-time statistics are the total number of rows and the number of modified rows that are automatically updated based on DML statements. When this variable is set to moderate (default), TiDB generates the execution plan based on real-time statistics. When this variable is set to determinate, TiDB does not use real-time statistics for generating the execution plan, which will make execution plans more stable.
For long-term stable OLTP workload, or if the user is affirmative on the existing execution plans, it is recommended to use the determinate mode to reduce the possibility of unexpected execution plan changes. Additionally, you can use the LOCK STATS to prevent the statistics from being modified and further stabilize the execution plan.

tidb_opt_ordering_index_selectivity_ratio New in v8.0.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Float
Default value: -1
Range: [-1, 1]
This variable controls the estimated number of rows for an index that matches the SQL statement ORDER BY when there are ORDER BY and LIMIT clauses in a SQL statement, but does not cover some filter conditions.
This addresses the same query patterns as the system variable tidb_opt_ordering_index_selectivity_threshold.
It differs in implementation by applying a ratio or percentage of the possible range that the qualified rows will be found.
A value of -1 (default) or less than 0 disables this ratio. Any value between 0 and 1 applies a ratio of 0% to 100% (for example, 0.5 corresponds to 50%).
In the following examples, the table t has a total of 1,000,000 rows. The same query is used, but different values for tidb_opt_ordering_index_selectivity_ratio are used. The query in the example contains a WHERE clause predicate that qualifies a small percentage of rows (9,000 out of 1,000,000). There is an index that supports the ORDER BY a (index ia), but the filter on b is not included in this index. Depending on the actual data distribution, the rows matching the WHERE clause and LIMIT 1 might be found as the first row accessed when scanning the non-filtering index, or at worst, after nearly all the rows have been processed.
Each example uses an index hint to demonstrate the impact on estRows. The final plan selection depends on the availability and cost of other plans.

The first example uses the default value -1, which uses the existing estimation formula. By default, a small percentage of rows are scanned for estimation before the qualified rows are found.

> SET SESSION tidb_opt_ordering_index_selectivity_ratio = -1;

> EXPLAIN SELECT * FROM t USE INDEX (ia) WHERE b <= 9000 ORDER BY a LIMIT 1;
+-----------------------------------+---------+-----------+-----------------------+---------------------------------+
| id                                | estRows | task      | access object         | operator info                   |
+-----------------------------------+---------+-----------+-----------------------+---------------------------------+
| Limit_12                          | 1.00    | root      |                       | offset:0, count:1               |
| └─Projection_22                   | 1.00    | root      |                       | test.t.a, test.t.b, test.t.c    |
|   └─IndexLookUp_21                | 1.00    | root      |                       |                                 |
|     ├─IndexFullScan_18(Build)     | 109.20  | cop[tikv] | table:t, index:ia(a)  | keep order:true                 |
|     └─Selection_20(Probe)         | 1.00    | cop[tikv] |                       | le(test.t.b, 9000)              |
|       └─TableRowIDScan_19         | 109.20  | cop[tikv] | table:t               | keep order:false                |
+-----------------------------------+---------+-----------+-----------------------+---------------------------------+

The second example uses 0, which assumes that 0% of rows will be scanned before the qualified rows are found.

> SET SESSION tidb_opt_ordering_index_selectivity_ratio = 0;

> EXPLAIN SELECT * FROM t USE INDEX (ia) WHERE b <= 9000 ORDER BY a LIMIT 1;
+-----------------------------------+---------+-----------+-----------------------+---------------------------------+
| id                                | estRows | task      | access object         | operator info                   |
+-----------------------------------+---------+-----------+-----------------------+---------------------------------+
| Limit_12                          | 1.00    | root      |                       | offset:0, count:1               |
| └─Projection_22                   | 1.00    | root      |                       | test.t.a, test.t.b, test.t.c    |
|   └─IndexLookUp_21                | 1.00    | root      |                       |                                 |
|     ├─IndexFullScan_18(Build)     | 1.00    | cop[tikv] | table:t, index:ia(a)  | keep order:true                 |
|     └─Selection_20(Probe)         | 1.00    | cop[tikv] |                       | le(test.t.b, 9000)              |
|       └─TableRowIDScan_19         | 1.00    | cop[tikv] | table:t               | keep order:false                |
+-----------------------------------+---------+-----------+-----------------------+---------------------------------+

The third example uses 0.1, which assumes that 10% of rows will be scanned before the qualified rows are found. This condition is highly selective, with only 1% of rows meeting the condition. Therefore, in the worst-case scenario, it might be necessary to scan 99% of rows before finding the 1% that qualify. 10% of that 99% is approximately 9.9%, which is reflected in the estRows.

> SET SESSION tidb_opt_ordering_index_selectivity_ratio = 0.1;

> EXPLAIN SELECT * FROM t USE INDEX (ia) WHERE b <= 9000 ORDER BY a LIMIT 1;
+-----------------------------------+----------+-----------+-----------------------+---------------------------------+
| id                                | estRows  | task      | access object         | operator info                   |
+-----------------------------------+----------+-----------+-----------------------+---------------------------------+
| Limit_12                          | 1.00     | root      |                       | offset:0, count:1               |
| └─Projection_22                   | 1.00     | root      |                       | test.t.a, test.t.b, test.t.c    |
|   └─IndexLookUp_21                | 1.00     | root      |                       |                                 |
|     ├─IndexFullScan_18(Build)     | 99085.21 | cop[tikv] | table:t, index:ia(a)  | keep order:true                 |
|     └─Selection_20(Probe)         | 1.00     | cop[tikv] |                       | le(test.t.b, 9000)              |
|       └─TableRowIDScan_19         | 99085.21 | cop[tikv] | table:t               | keep order:false                |
+-----------------------------------+----------+-----------+-----------------------+---------------------------------+

The fourth example uses 1.0, which assumes that 100% of rows will be scanned before the qualified rows are found.

> SET SESSION tidb_opt_ordering_index_selectivity_ratio = 1;

> EXPLAIN SELECT * FROM t USE INDEX (ia) WHERE b <= 9000 ORDER BY a LIMIT 1;
+-----------------------------------+-----------+-----------+-----------------------+---------------------------------+
| id                                | estRows   | task      | access object         | operator info                   |
+-----------------------------------+-----------+-----------+-----------------------+---------------------------------+
| Limit_12                          | 1.00      | root      |                       | offset:0, count:1               |
| └─Projection_22                   | 1.00      | root      |                       | test.t.a, test.t.b, test.t.c    |
|   └─IndexLookUp_21                | 1.00      | root      |                       |                                 |
|     ├─IndexFullScan_18(Build)     | 990843.14 | cop[tikv] | table:t, index:ia(a)  | keep order:true                 |
|     └─Selection_20(Probe)         | 1.00      | cop[tikv] |                       | le(test.t.b, 9000)              |
|       └─TableRowIDScan_19         | 990843.14 | cop[tikv] | table:t               | keep order:false                |
+-----------------------------------+-----------+-----------+-----------------------+---------------------------------+

The fifth example also uses 1.0, but adds a predicate on a, limiting the scan range in the worst-case scenario. This is because WHERE a <= 9000 matches the index, with approximately 9,000 rows would qualify. Given that the filter predicate on b is not in the index, all the approximately 9,000 rows are considered to be scanned before finding a row that matches b <= 9000.

> SET SESSION tidb_opt_ordering_index_selectivity_ratio = 1;

> EXPLAIN SELECT * FROM t USE INDEX (ia) WHERE a <= 9000 AND b <= 9000 ORDER BY a LIMIT 1;
+------------------------------------+---------+-----------+-----------------------+------------------------------------+
| id                                 | estRows | task      | access object         | operator info                      |
+------------------------------------+---------+-----------+-----------------------+------------------------------------+
| Limit_12                           | 1.00    | root      |                       | offset:0, count:1                  |
| └─Projection_22                    | 1.00    | root      |                       | test.t.a, test.t.b, test.t.c       |
|   └─IndexLookUp_21                 | 1.00    | root      |                       |                                    |
|     ├─IndexRangeScan_18(Build)     | 9074.99 | cop[tikv] | table:t, index:ia(a)  | range:[-inf,9000], keep order:true |
|     └─Selection_20(Probe)          | 1.00    | cop[tikv] |                       | le(test.t.b, 9000)                 |
|       └─TableRowIDScan_19          | 9074.99 | cop[tikv] | table:t               | keep order:false                   |
+------------------------------------+---------+-----------+-----------------------+------------------------------------+

tidb_opt_ordering_index_selectivity_threshold New in v7.0.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Float
Default value: 0
Range: [0, 1]
This variable is used to control how the optimizer selects an index when there are ORDER BY and LIMIT clauses with filter conditions in a SQL statement.
For such queries, the optimizer considers selecting the corresponding index to satisfy the ORDER BY and LIMIT clauses (even if this index does not satisfy any filter conditions). However, due to the complexity of data distribution, the optimizer might select a suboptimal index in this scenario.
This variable represents a threshold. When an index exists that can satisfy filtering conditions and its selectivity estimate is lower than this threshold, the optimizer will avoid selecting an index used to satisfy ORDER BY and LIMIT. Instead, it prioritizes an index that satisfies the filtering conditions.
For example, when the variable is set to 0, the optimizer maintains its default behavior; when it is set to 1, the optimizer always prioritizes selecting indexes that satisfy the filter conditions and avoids selecting indexes that satisfy both ORDER BY and LIMIT clauses.
In the following example, table t has a total of 1,000,000 rows. When using an index on column b, its estimated row count is approximately 8,748, so its selectivity estimate value is about 0.0087. By default, the optimizer selects an index on column a. However, after setting this variable to 0.01, since the selectivity of an index on column b (0.0087) is less than 0.01, the optimizer selects an index on column b.

> EXPLAIN SELECT * FROM t WHERE b <= 9000 ORDER BY a LIMIT 1;
+-----------------------------------+---------+-----------+----------------------+--------------------+
| id                                | estRows | task      | access object        | operator info      |
+-----------------------------------+---------+-----------+----------------------+--------------------+
| Limit_12                          | 1.00    | root      |                      | offset:0, count:1  |
| └─Projection_25                   | 1.00    | root      |                      | test.t.a, test.t.b |
|   └─IndexLookUp_24                | 1.00    | root      |                      |                    |
|     ├─IndexFullScan_21(Build)     | 114.30  | cop[tikv] | table:t, index:ia(a) | keep order:true    |
|     └─Selection_23(Probe)         | 1.00    | cop[tikv] |                      | le(test.t.b, 9000) |
|       └─TableRowIDScan_22         | 114.30  | cop[tikv] | table:t              | keep order:false   |
+-----------------------------------+---------+-----------+----------------------+--------------------+

> SET SESSION tidb_opt_ordering_index_selectivity_threshold = 0.01;

> EXPLAIN SELECT * FROM t WHERE b <= 9000 ORDER BY a LIMIT 1;
+----------------------------------+---------+-----------+----------------------+-------------------------------------+
| id                               | estRows | task      | access object        | operator info                       |
+----------------------------------+---------+-----------+----------------------+-------------------------------------+
| TopN_9                           | 1.00    | root      |                      | test.t.a, offset:0, count:1         |
| └─IndexLookUp_20                 | 1.00    | root      |                      |                                     |
|   ├─IndexRangeScan_17(Build)     | 8748.62 | cop[tikv] | table:t, index:ib(b) | range:[-inf,9000], keep order:false |
|   └─TopN_19(Probe)               | 1.00    | cop[tikv] |                      | test.t.a, offset:0, count:1         |
|     └─TableRowIDScan_18          | 8748.62 | cop[tikv] | table:t              | keep order:false                    |
+----------------------------------+---------+-----------+----------------------+-------------------------------------+

tidb_opt_prefer_range_scan New in v5.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Boolean
Default value: OFF
After you set the value of this variable to ON, the optimizer always prefers range scans over full table scans.
In the following example, before you enable tidb_opt_prefer_range_scan, the TiDB optimizer performs a full table scan. After you enable tidb_opt_prefer_range_scan, the optimizer selects an index range scan.

explain select * from t where age=5;
+-------------------------+------------+-----------+---------------+-------------------+
| id                      | estRows    | task      | access object | operator info     |
+-------------------------+------------+-----------+---------------+-------------------+
| TableReader_7           | 1048576.00 | root      |               | data:Selection_6  |
| └─Selection_6           | 1048576.00 | cop[tikv] |               | eq(test.t.age, 5) |
|   └─TableFullScan_5     | 1048576.00 | cop[tikv] | table:t       | keep order:false  |
+-------------------------+------------+-----------+---------------+-------------------+
3 rows in set (0.00 sec)

set session tidb_opt_prefer_range_scan = 1;

explain select * from t where age=5;
+-------------------------------+------------+-----------+-----------------------------+-------------------------------+
| id                            | estRows    | task      | access object               | operator info                 |
+-------------------------------+------------+-----------+-----------------------------+-------------------------------+
| IndexLookUp_7                 | 1048576.00 | root      |                             |                               |
| ├─IndexRangeScan_5(Build)     | 1048576.00 | cop[tikv] | table:t, index:idx_age(age) | range:[5,5], keep order:false |
| └─TableRowIDScan_6(Probe)     | 1048576.00 | cop[tikv] | table:t                     | keep order:false              |
+-------------------------------+------------+-----------+-----------------------------+-------------------------------+
3 rows in set (0.00 sec)

tidb_opt_prefix_index_single_scan New in v6.4.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Default value: ON
This variable controls whether the TiDB optimizer pushes down some filter conditions to the prefix index to avoid unnecessary table lookup and to improve query performance.
When this variable value is set to ON, some filter conditions are pushed down to the prefix index. Suppose that the col column is the index prefix column in a table. The col is null or col is not null condition in the query is handled as a filter condition on the index instead of a filter condition for the table lookup, so that unnecessary table lookup is avoided.

Usage example of tidb_opt_prefix_index_single_scan

Create a table with a prefix index:

CREATE TABLE t (a INT, b VARCHAR(10), c INT, INDEX idx_a_b(a, b(5)));

Disable tidb_opt_prefix_index_single_scan:

SET tidb_opt_prefix_index_single_scan = 'OFF';

For the following query, the execution plan uses the prefix index idx_a_b but requires a table lookup (the IndexLookUp operator appears).

EXPLAIN FORMAT='brief' SELECT COUNT(1) FROM t WHERE a = 1 AND b IS NOT NULL;
+-------------------------------+---------+-----------+------------------------------+-------------------------------------------------------+
| id                            | estRows | task      | access object                | operator info                                         |
+-------------------------------+---------+-----------+------------------------------+-------------------------------------------------------+
| HashAgg                       | 1.00    | root      |                              | funcs:count(Column#8)->Column#5                       |
| └─IndexLookUp                 | 1.00    | root      |                              |                                                       |
|   ├─IndexRangeScan(Build)     | 99.90   | cop[tikv] | table:t, index:idx_a_b(a, b) | range:[1 -inf,1 +inf], keep order:false, stats:pseudo |
|   └─HashAgg(Probe)            | 1.00    | cop[tikv] |                              | funcs:count(1)->Column#8                              |
|     └─Selection               | 99.90   | cop[tikv] |                              | not(isnull(test.t.b))                                 |
|       └─TableRowIDScan        | 99.90   | cop[tikv] | table:t                      | keep order:false, stats:pseudo                        |
+-------------------------------+---------+-----------+------------------------------+-------------------------------------------------------+
6 rows in set (0.00 sec)

Enable tidb_opt_prefix_index_single_scan:

SET tidb_opt_prefix_index_single_scan = 'ON';

After enabling this variable, for the following query, the execution plan uses the prefix index idx_a_b but does not require a table lookup.

EXPLAIN FORMAT='brief' SELECT COUNT(1) FROM t WHERE a = 1 AND b IS NOT NULL;
+--------------------------+---------+-----------+------------------------------+-------------------------------------------------------+
| id                       | estRows | task      | access object                | operator info                                         |
+--------------------------+---------+-----------+------------------------------+-------------------------------------------------------+
| StreamAgg                | 1.00    | root      |                              | funcs:count(Column#7)->Column#5                       |
| └─IndexReader            | 1.00    | root      |                              | index:StreamAgg                                       |
|   └─StreamAgg            | 1.00    | cop[tikv] |                              | funcs:count(1)->Column#7                              |
|     └─IndexRangeScan     | 99.90   | cop[tikv] | table:t, index:idx_a_b(a, b) | range:[1 -inf,1 +inf], keep order:false, stats:pseudo |
+--------------------------+---------+-----------+------------------------------+-------------------------------------------------------+
4 rows in set (0.00 sec)

tidb_opt_projection_push_down New in v6.1.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Boolean
Default value: ON. Before v8.3.0, the default value is OFF.
Specifies whether to allow the optimizer to push the Projection operator down to the TiKV coprocessor. When enabled, the optimizer might push the following three types of Projection operators down to TiKV:
- The top-level expressions of the operator are all JSON query functions or JSON value attribute functions. For example: SELECT JSON_EXTRACT(data, '$.name') FROM users;.
- The top-level expressions of the operator include a mix of JSON query functions or JSON value attribute functions, and direct column reads. For example: SELECT JSON_DEPTH(data), name FROM users;.
- The top-level expressions of the operator are all direct column reads, and the number of output columns is less than the number of input columns. For example: SELECT name FROM users;.
The final decision to push down a Projection operator also depends on the optimizer's comprehensive evaluation of query cost.
For TiDB clusters that are upgraded from a version earlier than v8.3.0 to v8.3.0 or later, the default value of this variable is OFF.

tidb_opt_range_max_size New in v6.4.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Default value: 67108864 (64 MiB)
Scope: [0, 9223372036854775807]
Unit: Bytes
This variable is used to set the upper limit of memory usage for the optimizer to build scan ranges. When the variable value is 0, there is no memory limit for building scan ranges. If building exact scan ranges consumes memory that exceeds the limit, the optimizer uses more relaxed scan ranges (such as [[NULL,+inf]]). If the execution plan does not use exact scan ranges, you can increase the value of this variable to let the optimizer build exact scan ranges.

The usage example of this variable is as follows:

tidb_opt_range_max_size usage examples

View the default value of this variable. From the result, you can see that the optimizer uses up to 64 MiB of memory to build scan ranges.

SELECT @@tidb_opt_range_max_size;

+----------------------------+
| @@tidb_opt_range_max_size |
+----------------------------+
| 67108864                   |
+----------------------------+
1 row in set (0.01 sec)

EXPLAIN SELECT * FROM t use index (idx) WHERE a IN (10,20,30) AND b IN (40,50,60);

In the 64 MiB memory upper limit, the optimizer builds the following exact scan ranges [10 40,10 40], [10 50,10 50], [10 60,10 60], [20 40,20 40], [20 50,20 50], [20 60,20 60], [30 40,30 40], [30 50,30 50], [30 60,30 60], as shown in the following execution plan result.

+-------------------------------+---------+-----------+--------------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| id                            | estRows | task      | access object            | operator info                                                                                                                                                               |
+-------------------------------+---------+-----------+--------------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| IndexLookUp_7                 | 0.90    | root      |                          |                                                                                                                                                                             |
| ├─IndexRangeScan_5(Build)     | 0.90    | cop[tikv] | table:t, index:idx(a, b) | range:[10 40,10 40], [10 50,10 50], [10 60,10 60], [20 40,20 40], [20 50,20 50], [20 60,20 60], [30 40,30 40], [30 50,30 50], [30 60,30 60], keep order:false, stats:pseudo |
| └─TableRowIDScan_6(Probe)     | 0.90    | cop[tikv] | table:t                  | keep order:false, stats:pseudo                                                                                                                                              |
+-------------------------------+---------+-----------+--------------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
3 rows in set (0.00 sec)

Now set the upper limit of memory usage for the optimizer to build scan ranges to 1500 bytes.

SET @@tidb_opt_range_max_size = 1500;

Query OK, 0 rows affected (0.00 sec)

EXPLAIN SELECT * FROM t USE INDEX (idx) WHERE a IN (10,20,30) AND b IN (40,50,60);

In the 1500-byte memory limit, the optimizer builds more relaxed scan ranges [10,10], [20,20], [30,30], and uses a warning to inform the user that the memory usage required to build exact scan ranges exceeds the limit of tidb_opt_range_max_size.

+-------------------------------+---------+-----------+--------------------------+-----------------------------------------------------------------+
| id                            | estRows | task      | access object            | operator info                                                   |
+-------------------------------+---------+-----------+--------------------------+-----------------------------------------------------------------+
| IndexLookUp_8                 | 0.09    | root      |                          |                                                                 |
| ├─Selection_7(Build)          | 0.09    | cop[tikv] |                          | in(test.t.b, 40, 50, 60)                                        |
| │ └─IndexRangeScan_5          | 30.00   | cop[tikv] | table:t, index:idx(a, b) | range:[10,10], [20,20], [30,30], keep order:false, stats:pseudo |
| └─TableRowIDScan_6(Probe)     | 0.09    | cop[tikv] | table:t                  | keep order:false, stats:pseudo                                  |
+-------------------------------+---------+-----------+--------------------------+-----------------------------------------------------------------+
4 rows in set, 1 warning (0.00 sec)

SHOW WARNINGS;

+---------+------+---------------------------------------------------------------------------------------------------------------------------------------------+
| Level   | Code | Message                                                                                                                                     |
+---------+------+---------------------------------------------------------------------------------------------------------------------------------------------+
| Warning | 1105 | Memory capacity of 1500 bytes for 'tidb_opt_range_max_size' exceeded when building ranges. Less accurate ranges such as full range are chosen |
+---------+------+---------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

Then set the upper limit of memory usage to 100 bytes:

set @@tidb_opt_range_max_size = 100;

Query OK, 0 rows affected (0.00 sec)

EXPLAIN SELECT * FROM t USE INDEX (idx) WHERE a IN (10,20,30) AND b IN (40,50,60);

In the 100-byte memory limit, the optimizer chooses IndexFullScan, and uses a warning to inform the user that the memory required to build exact scan ranges exceeds the limit of tidb_opt_range_max_size.

+-------------------------------+----------+-----------+--------------------------+----------------------------------------------------+
| id                            | estRows  | task      | access object            | operator info                                      |
+-------------------------------+----------+-----------+--------------------------+----------------------------------------------------+
| IndexLookUp_8                 | 8000.00  | root      |                          |                                                    |
| ├─Selection_7(Build)          | 8000.00  | cop[tikv] |                          | in(test.t.a, 10, 20, 30), in(test.t.b, 40, 50, 60) |
| │ └─IndexFullScan_5           | 10000.00 | cop[tikv] | table:t, index:idx(a, b) | keep order:false, stats:pseudo                     |
| └─TableRowIDScan_6(Probe)     | 8000.00  | cop[tikv] | table:t                  | keep order:false, stats:pseudo                     |
+-------------------------------+----------+-----------+--------------------------+----------------------------------------------------+
4 rows in set, 1 warning (0.00 sec)

SHOW WARNINGS;

+---------+------+---------------------------------------------------------------------------------------------------------------------------------------------+
| Level   | Code | Message                                                                                                                                     |
+---------+------+---------------------------------------------------------------------------------------------------------------------------------------------+
| Warning | 1105 | Memory capacity of 100 bytes for 'tidb_opt_range_max_size' exceeded when building ranges. Less accurate ranges such as full range are chosen |
+---------+------+---------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

tidb_opt_scan_factor

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Float
Range: [0, 2147483647]
Default value: 1.5
Indicates the cost for TiKV to scan one row of data from the disk in ascending order. This variable is internally used in the Cost Model, and it is NOT recommended to modify its value.

tidb_opt_seek_factor

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Float
Range: [0, 2147483647]
Default value: 20
Indicates the start-up cost for TiDB to request data from TiKV. This variable is internally used in the Cost Model, and it is NOT recommended to modify its value.

tidb_opt_skew_distinct_agg New in v6.2.0

Note

The query performance optimization by enabling this variable is effective only for TiFlash.

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Boolean
Default value: OFF
This variable sets whether the optimizer rewrites the aggregate functions with DISTINCT to the two-level aggregate functions, such as rewriting SELECT b, COUNT(DISTINCT a) FROM t GROUP BY b to SELECT b, COUNT(a) FROM (SELECT b, a FROM t GROUP BY b, a) t GROUP BY b. When the aggregation column has serious skew and the DISTINCT column has many different values, this rewriting can avoid the data skew in the query execution and improve the query performance.

tidb_opt_three_stage_distinct_agg New in v6.3.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Boolean
Default value: ON
This variable specifies whether to rewrite a COUNT(DISTINCT) aggregation into a three-stage aggregation in MPP mode.
This variable currently applies to an aggregation that only contains one COUNT(DISTINCT).

tidb_opt_tiflash_concurrency_factor

Scope: SESSION | GLOBAL
Persists to cluster: YES
Applies to hint SET_VAR: Yes
Type: Float
Range: [0, 2147483647]
Default value: 24.0
Indicates the concurrency number of TiFlash computation. This variable is internally used in the Cost Model, and it is NOT recommended to modify its value.

tidb_opt_use_invisible_indexes New in v8.0.0

Scope: SESSION
Applies to hint SET_VAR: Yes
Type: Boolean
Default value: OFF
This variable controls whether the optimizer can select invisible indexes for query optimization in the current session. Invisible indexes are maintained by DML statements, but will not be used by the query optimizer. This is useful in scenarios where you want to double-check before removing an index permanently. When the variable is set to ON, the optimizer can select invisible indexes for query optimization in the session.

tidb_opt_write_row_id

Note

This TiDB variable is not applicable to TiDB Cloud.

Scope: SESSION
Applies to hint SET_VAR: Yes
Type: Boolean
Default value: OFF
This variable is used to control whether to allow INSERT, REPLACE, and UPDATE statements to operate on the _tidb_rowid column. This variable can be used only when you import data using TiDB tools.

tidb_optimizer_selectivity_level

Scope: SESSION
Applies to hint SET_VAR: Yes
Type: Integer
Default value: 0
Range: [0, 2147483647]
This variable controls the iteration of the optimizer's estimation logic. After changing the value of this variable, the estimation logic of the optimizer will change greatly. Currently, 0 is the only valid value. It is not recommended to set it to other values.

tidb_partition_prune_mode New in v5.1

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Enumeration
Default value: dynamic
Possible values: static, dynamic, static-only, dynamic-only
Specifies whether to use dynamic or static mode for partitioned tables. Note that dynamic partitioning is effective only after full table-level statistics, or GlobalStats, are collected. Before GlobalStats are collected, TiDB will use the static mode instead. For detailed information about GlobalStats, see Collect statistics of partitioned tables in dynamic pruning mode. For details about the dynamic pruning mode, see Dynamic Pruning Mode for Partitioned Tables.

tidb_persist_analyze_options New in v5.4.0

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: ON
This variable controls whether to enable the ANALYZE configuration persistence feature.

tidb_pessimistic_txn_fair_locking New in v7.0.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: ON
Determines whether to use enhanced pessimistic locking wake-up model for pessimistic transactions. This model strictly controls the wake-up order of pessimistic transactions in the pessimistic locking single-point conflict scenarios to avoid unnecessary wake-ups. It greatly reduces the uncertainty brought by the randomness of the existing wake-up mechanism. If you encounter frequent single-point pessimistic locking conflicts in your business scenario (such as frequent updates to the same row of data), and thus cause frequent statement retries, high tail latency, or even occasional pessimistic lock retry limit reached errors, you can try to enable this variable to solve the problem.
This variable is disabled by default for TiDB clusters that are upgraded from versions earlier than v7.0.0 to v7.0.0 or later versions.

Note

Depending on the specific business scenario, enabling this option might cause a certain degree of throughput reduction (average latency increase) for transactions with frequent lock conflicts.
This option only takes effect on statements that need to lock a single key. If a statement needs to lock multiple rows at the same time, this option will not take effect on such statements.
This feature is introduced in v6.6.0 by the tidb_pessimistic_txn_aggressive_locking variable, which is disabled by default.

tidb_placement_mode New in v6.0.0

Note

This variable is read-only for TiDB Serverless.

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Enumeration
Default value: STRICT
Possible values: STRICT, IGNORE
This variable controls whether DDL statements ignore the placement rules specified in SQL. When the variable value is IGNORE, all placement rule options are ignored.
It is intended to be used by logical dump/restore tools to ensure that tables can always be created even if invalid placement rules are assigned. This is similar to how mysqldump writes SET FOREIGN_KEY_CHECKS=0; to the start of every dump file.

`tidb_plan_cache_invalidation_on_fresh_stats` New in v7.1.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: ON
This variable controls whether to invalidate the plan cache automatically when statistics on related tables are updated.
After enabling this variable, plan cache can make use of statistics more sufficiently to generate execution plans. For example:
- If execution plans are generated before statistics are available, plan cache re-generates execution plans once the statistics are available.
- If the data distribution of a table changes, causing the previously optimal execution plan to become non-optimal, plan cache re-generates execution plans after the statistics are re-collected.
This variable is disabled by default for TiDB clusters that are upgraded from a version earlier than v7.1.0 to v7.1.0 or later.

`tidb_plan_cache_max_plan_size` New in v7.1.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Default value: 2097152 (which is 2 MiB)
Range: [0, 9223372036854775807], in bytes. The memory format with the units "KiB|MiB|GiB|TiB" is also supported. 0 means no limit.
This variable controls the maximum size of a plan that can be cached in prepared or non-prepared plan cache. If the size of a plan exceeds this value, the plan will not be cached. For more details, see Memory management of prepared plan cache and Non-prepared plan cache.

tidb_pprof_sql_cpu New in v4.0

Note

This TiDB variable is not applicable to TiDB Cloud.

Scope: GLOBAL
Persists to cluster: No, only applicable to the current TiDB instance that you are connecting to.
Applies to hint SET_VAR: No
Type: Integer
Default value: 0
Range: [0, 1]
This variable is used to control whether to mark the corresponding SQL statement in the profile output to identify and troubleshoot performance issues.

tidb_prefer_broadcast_join_by_exchange_data_size New in v7.1.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Default value: OFF
This variable controls whether to use the algorithm with the minimum overhead of network transmission when TiDB selects the MPP Hash Join algorithm. If this variable is enabled, TiDB estimates the size of the data to be exchanged in the network using Broadcast Hash Join and Shuffled Hash Join respectively, and then chooses the one with the smaller size.
tidb_broadcast_join_threshold_count and tidb_broadcast_join_threshold_size will not take effect after this variable is enabled.

tidb_prepared_plan_cache_memory_guard_ratio New in v6.1.0

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Float
Default value: 0.1
Range: [0, 1]
The threshold at which the prepared plan cache triggers a memory protection mechanism. For details, see Memory management of Prepared Plan Cache.
This setting was previously a tidb.toml option (prepared-plan-cache.memory-guard-ratio), but changed to a system variable starting from TiDB v6.1.0.

tidb_prepared_plan_cache_size New in v6.1.0

Warning

Starting from v7.1.0, this variable is deprecated. Instead, use tidb_session_plan_cache_size for setting.

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 100
Range: [1, 100000]
The maximum number of plans that can be cached in a session. For details, see Memory management of Prepared Plan Cache.
This setting was previously a tidb.toml option (prepared-plan-cache.capacity), but changed to a system variable starting from TiDB v6.1.0.

tidb_projection_concurrency

Warning

Since v5.0, this variable is deprecated. Instead, use tidb_executor_concurrency for setting.

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: -1
Range: [-1, 256]
Unit: Threads
This variable is used to set the concurrency of the Projection operator.
A value of -1 means that the value of tidb_executor_concurrency will be used instead.

tidb_query_log_max_len

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 4096 (4 KiB)
Range: [0, 1073741824]
Unit: Bytes
The maximum length of the SQL statement output. When the output length of a statement is larger than the tidb_query_log_max_len value, the statement is truncated to output.
This setting was previously also available as a tidb.toml option (log.query-log-max-len), but is only a system variable starting from TiDB v6.1.0.

tidb_rc_read_check_ts New in v6.0.0

Warning

This feature is incompatible with replica-read. Do not enable tidb_rc_read_check_ts and replica-read at the same time.
If your client uses a cursor, it is not recommended to enable tidb_rc_read_check_ts in case that the previous batch of returned data has already been used by the client and the statement eventually fails.
Starting from v7.0.0, this variable is no longer valid for the cursor fetch read mode that uses the prepared statement protocol.

Scope: GLOBAL
Persists to cluster: No, only applicable to the current TiDB instance that you are connecting to.
Applies to hint SET_VAR: No
Type: Boolean
Default value: OFF
This variable is used to optimize the timestamp acquisition, which is suitable for scenarios with read-committed isolation level where read-write conflicts are rare. Enabling this variable can avoid the latency and cost of getting the global timestamp, and can optimize the transaction-level read latency.
If read-write conflicts are severe, enabling this feature will increase the cost and latency of getting the global timestamp, and might cause performance regression. For details, see Read Committed isolation level.

tidb_rc_write_check_ts New in v6.3.0

Warning

This feature is currently incompatible with replica-read. After this variable is enabled, all requests sent by the client cannot use replica-read. Therefore, do not enable tidb_rc_write_check_ts and replica-read at the same time.

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: OFF
This variable is used to optimize the acquisition of timestamps and is suitable for scenarios with few point-write conflicts in READ-COMMITTED isolation level of pessimistic transactions. Enabling this variable can avoid the latency and overhead brought by obtaining the global timestamps during the execution of point-write statements. Currently, this variable is applicable to three types of point-write statements: UPDATE, DELETE, and SELECT ...... FOR UPDATE. A point-write statement refers to a write statement that uses the primary key or unique key as a filter condition and the final execution operator contains POINT-GET.
If the point-write conflicts are severe, enabling this variable will increase extra overhead and latency, resulting in performance regression. For details, see Read Committed isolation level.

tidb_read_consistency New in v5.4.0

Scope: SESSION
Applies to hint SET_VAR: Yes
Type: String
Default value: strict
This variable is used to control the read consistency for an auto-commit read statement.
If the variable value is set to weak, the locks encountered by the read statement are skipped directly and the read execution might be faster, which is the weak consistency read mode. However, the transaction semantics (such as atomicity) and distributed consistency (such as linearizability) are not guaranteed.
For user scenarios where the auto-commit read needs to return fast and weak consistency read results are acceptable, you can use the weak consistency read mode.

tidb_read_staleness New in v5.4.0

Scope: SESSION
Applies to hint SET_VAR: Yes
Type: Integer
Default value: 0
Range: [-2147483648, 0]
This variable is used to set the time range of historical data that TiDB can read in the current session. After setting the value, TiDB selects a timestamp as new as possible from the range allowed by this variable, and all subsequent read operations are performed against this timestamp. For example, if the value of this variable is set to -5, on the condition that TiKV has the corresponding historical version's data, TiDB selects a timestamp as new as possible within a 5-second time range.

tidb_record_plan_in_slow_log

Note

This TiDB variable is not applicable to TiDB Cloud.

Scope: GLOBAL
Persists to cluster: No, only applicable to the current TiDB instance that you are connecting to.
Applies to hint SET_VAR: No
Type: Boolean
Default value: ON
This variable is used to control whether to include the execution plan of slow queries in the slow log.

tidb_redact_log

Note

This TiDB variable is not applicable to TiDB Cloud.

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Enumeration
Default value: OFF
Possible values: OFF, ON, MARKER
This variable controls whether to hide the user information in the SQL statement being recorded into the TiDB log and slow log.
The default value is OFF, which means that the user information is not processed in any way.
When you set the variable to ON, the user information is hidden. For example, if the executed SQL statement is INSERT INTO t VALUES (1,2), the statement is recorded as INSERT INTO t VALUES (?,?) in the log.
When you set the variable to MARKER, the user information is wrapped in ‹ ›. For example, if the executed SQL statement is INSERT INTO t VALUES (1,2), the statement is recorded as INSERT INTO t VALUES (‹1›,‹2›) in the log. If user data contains ‹ or ›, ‹ is escaped as ‹‹, and › is escaped as ››. Based on the marked logs, you can decide whether to desensitize the marked information when the logs are displayed.

tidb_regard_null_as_point New in v5.4.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: ON
This variable controls whether the optimizer can use a query condition including null equivalence as a prefix condition for index access.
This variable is enabled by default. When it is enabled, the optimizer can reduce the volume of index data to be accessed, which accelerates query execution. For example, if a query involves multiple-column indexes index(a, b) and the query condition contains a<=>null and b=1, the optimizer can use both a<=>null and b=1 in the query condition for index access. If the variable is disabled, because a<=>null and b=1 includes the null equivalence condition, the optimizer does not use b=1 for index access.

tidb_remove_orderby_in_subquery New in v6.1.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: Before v7.2.0, the default value is OFF. Starting from v7.2.0, the default value is ON.
Specifies whether to remove ORDER BY clause in a subquery.
In the ISO/IEC SQL standard, ORDER BY is mainly used to sort the results of top-level queries. For subqueries, the standard does not require that the results be sorted by ORDER BY.
To sort subquery results, you can usually handle it in the outer query, such as using the window function or using ORDER BY again in the outer query. Doing so ensures the order of the final result set.

tidb_replica_read New in v4.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Enumeration
Default value: leader
Possible values: leader, follower, leader-and-follower, prefer-leader, closest-replicas, closest-adaptive, and learner. The learner value is introduced in v6.6.0.
This variable is used to control where TiDB reads data.
For more details about usage and implementation, see Follower read.

tidb_restricted_read_only New in v5.2.0

Note

This TiDB variable is not applicable to TiDB Cloud.

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: OFF
tidb_restricted_read_only and tidb_super_read_only behave similarly. In most cases, you should use tidb_super_read_only only.
Users with the SUPER or SYSTEM_VARIABLES_ADMIN privilege can modify this variable. However, if the Security Enhanced Mode is enabled, the additional RESTRICTED_VARIABLES_ADMIN privilege is required to read or modify this variable.
tidb_restricted_read_only affects tidb_super_read_only in the following cases:
- Setting tidb_restricted_read_only to ON will update tidb_super_read_only to ON.
- Setting tidb_restricted_read_only to OFF leaves tidb_super_read_only unchanged.
- If tidb_restricted_read_only is ON, tidb_super_read_only cannot be set to OFF.
For DBaaS providers of TiDB, if a TiDB cluster is a downstream database of another database, to make the TiDB cluster read-only, you might need to use tidb_restricted_read_only with Security Enhanced Mode enabled, which prevents your customers from using tidb_super_read_only to make the cluster writable. To achieve this, you need to enable Security Enhanced Mode, use an admin user with the SYSTEM_VARIABLES_ADMIN and RESTRICTED_VARIABLES_ADMIN privileges to control tidb_restricted_read_only, and let your database users use the root user with the SUPER privilege to control tidb_super_read_only only.
This variable controls the read-only status of the entire cluster. When the variable is ON, all TiDB servers in the entire cluster are in the read-only mode. In this case, TiDB only executes the statements that do not modify data, such as SELECT, USE, and SHOW. For other statements such as INSERT and UPDATE, TiDB rejects executing those statements in the read-only mode.
Enabling the read-only mode using this variable only ensures that the entire cluster finally enters the read-only status. If you have changed the value of this variable in a TiDB cluster but the change has not yet propagated to other TiDB servers, the un-updated TiDB servers are still not in the read-only mode.
TiDB checks the read-only flag before SQL statements are executed. Since v6.2.0, the flag is also checked before SQL statements are committed. This helps prevent the case where long-running auto commit statements might modify data after the server has been placed in read-only mode.
When this variable is enabled, TiDB handles the uncommitted transactions in the following ways:
- For uncommitted read-only transactions, you can commit the transactions normally.
- For uncommitted transactions that are not read-only, SQL statements that perform write operations in these transactions are rejected.
- For uncommitted read-only transactions with modified data, the commit of these transactions is rejected.
After the read-only mode is enabled, all users (including the users with the SUPER privilege) cannot execute the SQL statements that might write data unless the user is explicitly granted the RESTRICTED_REPLICA_WRITER_ADMIN privilege.

tidb_request_source_type New in v7.4.0

Scope: SESSION
Applies to hint SET_VAR: No
Type: String
Default value: ""
Possible values: "ddl", "stats", "br", "lightning", "background"
This variable is used to explicitly specify the task type for the current session, which is identified and controlled by Resource Control. For example: SET @@tidb_request_source_type = "background".

tidb_resource_control_strict_mode New in v8.2.0

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: ON
This variable controls whether privilege control is applied to the SET RESOURCE GROUP statement and the RESOURCE_GROUP() optimizer hint. When this system variable is set to ON, you need to have the SUPER or RESOURCE_GROUP_ADMIN or RESOURCE_GROUP_USER privilege to change the bound resource group of the current session or current statement via these two ways. When it is set to OFF, none of these privileges are required, and the behavior is the same as earlier TiDB versions without this variable.
When you upgrade your TiDB cluster from an earlier version to v8.2.0 or later, the default value of this variable is set to OFF, which means this feature is disabled by default.

tidb_retry_limit

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 10
Range: [-1, 9223372036854775807]
This variable is used to set the maximum number of the retries for optimistic transactions. When a transaction encounters retryable errors (such as transaction conflicts, very slow transaction commit, or table schema changes), this transaction is re-executed according to this variable. Note that setting tidb_retry_limit to 0 disables the automatic retry. This variable only applies to optimistic transactions, not to pessimistic transactions.

tidb_row_format_version

Note

This TiDB variable is not applicable to TiDB Cloud.

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 2
Range: [1, 2]
Controls the format version of the newly saved data in the table. In TiDB v4.0, the new storage row format version 2 is used by default to save new data.
If you upgrade from a TiDB version earlier than v4.0.0 to v4.0.0 or later versions, the format version is not changed, and TiDB continues to use the old format of version 1 to write data to the table, which means that only newly created clusters use the new data format by default.
Note that modifying this variable does not affect the old data that has been saved, but applies the corresponding version format only to the newly written data after modifying this variable.

tidb_runtime_filter_mode New in v7.2.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Enumeration
Default value: OFF
Possible values: OFF, LOCAL
Controls the mode of Runtime Filter, that is, the relationship between the Filter Sender operator and Filter Receiver operator. There are two modes: OFF and LOCAL. OFF means disabling Runtime Filter. LOCAL means enabling Runtime Filter in the local mode. For more information, see Runtime Filter mode.

tidb_runtime_filter_type New in v7.2.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Enumeration
Default value: IN
Possible values: IN
Controls the type of predicate used by the generated Filter operator. Currently, only one type is supported: IN. For more information, see Runtime Filter type.

tidb_scatter_region

Note

This variable is read-only for TiDB Serverless.

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: OFF
By default, Regions are split for a new table when it is being created in TiDB. After this variable is enabled, the newly split Regions are scattered immediately during the execution of the CREATE TABLE statement. This applies to the scenario where data need to be written in batches right after the tables are created in batches, because the newly split Regions can be scattered in TiKV beforehand and do not have to wait to be scheduled by PD. To ensure the continuous stability of writing data in batches, the CREATE TABLE statement returns success only after the Regions are successfully scattered. This makes the statement's execution time multiple times longer than that when you disable this variable.
Note that if SHARD_ROW_ID_BITS and PRE_SPLIT_REGIONS have been set when a table is created, the specified number of Regions are evenly split after the table creation.

tidb_schema_cache_size New in v8.0.0

Warning

The feature controlled by this variable is not yet effective in the current TiDB version. Do not change the default value.

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 0
Range: 0 or [536870912, 9223372036854775807]
This variable controls the size of the schema cache in TiDB. The unit is byte. The default value is 0, which means the cache limit feature is disabled. To enable this feature, you need to set a value within the range [536870912, 9223372036854775807]. TiDB will use this value as the maximum available memory limit and apply the Least Recently Used (LRU) algorithm to cache the required tables, effectively reducing the memory used by schema information.

tidb_schema_version_cache_limit New in v7.4.0

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Default value: 16
Range: [2, 255]
This variable limits how many historical schema versions can be cached in a TiDB instance. The default value is 16, which means that TiDB caches 16 historical schema versions by default.
Generally, you do not need to modify this variable. When the Stale Read feature is used and DDL operations are executed very frequently, it will cause the schema version to change very frequently. Consequently, when Stale Read tries to obtain schema information from a snapshot, it might take a lot of time to rebuild the information due to schema cache misses. In this case, you can increase the value of tidb_schema_version_cache_limit (for example, 32) to avoid the problem of schema cache misses.
Modifying this variable causes the memory usage of TiDB to increase slightly. Monitor the memory usage of TiDB to avoid OOM problems.

tidb_server_memory_limit New in v6.4.0

Note

This variable is read-only for TiDB Serverless.

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Default value: 80%
Range:
- You can set the value in the percentage format, which means the percentage of the memory usage relative to the total memory. The value range is [1%, 99%].
- You can also set the value in memory size. The value range is 0 and [536870912, 9223372036854775807] in bytes. The memory format with the units "KiB|MiB|GiB|TiB" is supported. 0 means no memory limit.
- If this variable is set to a memory size that is less than 512 MiB but not 0, TiDB uses 512 MiB as the actual size.
This variable specifies the memory limit for a TiDB instance. When the memory usage of TiDB reaches the limit, TiDB cancels the currently running SQL statement with the highest memory usage. After the SQL statement is successfully canceled, TiDB tries to call Golang GC to immediately reclaim memory to relieve memory stress as soon as possible.
Only the SQL statements with more memory usage than the tidb_server_memory_limit_sess_min_size limit are selected as the SQL statements to be canceled first.
Currently, TiDB cancels only one SQL statement at a time. After TiDB completely cancels a SQL statement and recovers resources, if the memory usage is still greater than the limit set by this variable, TiDB starts the next cancel operation.

tidb_server_memory_limit_gc_trigger New in v6.4.0

Note

This variable is read-only for TiDB Serverless.

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Default value: 70%
Range: [50%, 99%]
The threshold at which TiDB tries to trigger GC. When the memory usage of TiDB reaches the value of tidb_server_memory_limit * the value of tidb_server_memory_limit_gc_trigger, TiDB will actively trigger a Golang GC operation. Only one GC operation will be triggered in one minute.

tidb_server_memory_limit_sess_min_size New in v6.4.0

Note

This variable is read-only for TiDB Serverless.

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Default value: 134217728 (which is 128 MiB)
Range: [128, 9223372036854775807], in bytes. The memory format with the units "KiB|MiB|GiB|TiB" is also supported.
After you enable the memory limit, TiDB will terminate the SQL statement with the highest memory usage on the current instance. This variable specifies the minimum memory usage of the SQL statement to be terminated. If the memory usage of a TiDB instance that exceeds the limit is caused by too many sessions with low memory usage, you can properly lower the value of this variable to allow more sessions to be canceled.

tidb_service_scope New in v7.4.0

Note

This TiDB variable is not applicable to TiDB Cloud.

Scope: GLOBAL
Persists to cluster: No, only applicable to the current TiDB instance that you are connecting to.
Applies to hint SET_VAR: No
Type: String
Default value: ""
Optional value: a string with a length of up to 64 characters. Valid characters include digits 0-9, letters a-zA-Z, underscores _, and hyphens -.
This variable is an instance-level system variable. You can use it to control the service scope of each TiDB node under the TiDB Distributed eXecution Framework (DXF). The DXF determines which TiDB nodes can be scheduled to execute distributed tasks based on the value of this variable. For specific rules, see Task scheduling.

tidb_session_alias New in v7.4.0

Scope: SESSION
Persists to cluster: No
Applies to hint SET_VAR: Yes
Default value: ""
You can use this variable to customize the value of the session_alias column in the logs related to the current session, which helps identify the session in troubleshooting. This setting affects the logs of multiple nodes involved in the statement execution (including TiKV). The maximum length of this variable is limited to 64 characters, and any characters exceeding the length will be truncated automatically. Spaces at the end of the value will also be removed automatically.

tidb_session_plan_cache_size New in v7.1.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 100
Range: [1, 100000]
This variable controls the maximum number of plans that can be cached. Prepared plan cache and non-prepared plan cache share the same cache.
When you upgrade from an earlier version to a v7.1.0 or later version, this variable remains the same value as tidb_prepared_plan_cache_size

tidb_shard_allocate_step New in v5.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 9223372036854775807
Range: [1, 9223372036854775807]
This variable controls the maximum number of continuous IDs to be allocated for the AUTO_RANDOM or SHARD_ROW_ID_BITS attribute. Generally, AUTO_RANDOM IDs or the SHARD_ROW_ID_BITS annotated row IDs are incremental and continuous in one transaction. You can use this variable to solve the hotspot issue in large transaction scenarios.

tidb_simplified_metrics

Note

This variable is read-only for TiDB Serverless.

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: OFF
When this variable is enabled, TiDB does not collect or record the metrics that are not used in the Grafana panels.

tidb_skip_ascii_check New in v5.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: OFF
This variable is used to set whether to skip ASCII validation.
Validating ASCII characters affects the performance. When you are sure that the input characters are valid ASCII characters, you can set the variable value to ON.

tidb_skip_isolation_level_check

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: OFF
After this switch is enabled, if an isolation level unsupported by TiDB is assigned to tx_isolation, no error is reported. This helps improve compatibility with applications that set (but do not depend on) a different isolation level.

tidb> set tx_isolation='serializable';
ERROR 8048 (HY000): The isolation level 'serializable' is not supported. Set tidb_skip_isolation_level_check=1 to skip this error
tidb> set tidb_skip_isolation_level_check=1;
Query OK, 0 rows affected (0.00 sec)

tidb> set tx_isolation='serializable';
Query OK, 0 rows affected, 1 warning (0.00 sec)

tidb_skip_missing_partition_stats New in v7.3.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: ON
When accessing a partitioned table in dynamic pruning mode, TiDB aggregates the statistics of each partition to generate GlobalStats. This variable controls the generation of GlobalStats when partition statistics are missing.
- If this variable is ON, TiDB skips missing partition statistics when generating GlobalStats so the generation of GlobalStats is not affected.
- If this variable is OFF, TiDB stops generating GlobalStats when it detects any missing partition statistics.

tidb_skip_utf8_check

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: OFF
This variable is used to set whether to skip UTF-8 validation.
Validating UTF-8 characters affects the performance. When you are sure that the input characters are valid UTF-8 characters, you can set the variable value to ON.

Note

If the character check is skipped, TiDB might fail to detect illegal UTF-8 characters written by the application, cause decoding errors when ANALYZE is executed, and introduce other unknown encoding issues. If your application cannot guarantee the validity of the written string, it is not recommended to skip the character check.

tidb_slow_log_threshold

Note

This TiDB variable is not applicable to TiDB Cloud.

Scope: GLOBAL
Persists to cluster: No, only applicable to the current TiDB instance that you are connecting to.
Applies to hint SET_VAR: No
Type: Integer
Default value: 300
Range: [-1, 9223372036854775807]
Unit: Milliseconds
This variable outputs the threshold value of the time consumed by the slow log, and is set to 300 milliseconds by default. When the time consumed by a query is larger than this value, this query is considered as a slow query and its log is output to the slow query log. Note that when the output level of log.level is "debug", all queries are recorded in the slow query log, regardless of the setting of this variable.

tidb_slow_query_file

Note

This TiDB variable is not applicable to TiDB Cloud.

Scope: SESSION
Applies to hint SET_VAR: No
Default value: ""
When INFORMATION_SCHEMA.SLOW_QUERY is queried, only the slow query log name set by slow-query-file in the configuration file is parsed. The default slow query log name is "tidb-slow.log". To parse other logs, set the tidb_slow_query_file session variable to a specific file path, and then query INFORMATION_SCHEMA.SLOW_QUERY to parse the slow query log based on the set file path.

For details, see Identify Slow Queries.

tidb_snapshot

Scope: SESSION
Applies to hint SET_VAR: No
Default value: ""
This variable is used to set the time point at which the data is read by the session. For example, when you set the variable to "2017-11-11 20:20:20" or a TSO number like "400036290571534337", the current session reads the data of this moment.

tidb_source_id New in v6.5.0

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 1
Range: [1, 15]

This variable is used to configure the different cluster IDs in a bi-directional replication cluster.

tidb_stats_cache_mem_quota New in v6.1.0

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Unit: Byte
Default value: 0, which means that the memory quota is automatically set to half of the total memory size of the TiDB instance.
Range: [0, 1099511627776]
This variable sets the memory quota for the TiDB statistics cache.

tidb_stats_load_pseudo_timeout New in v5.4.0

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: ON
This variable controls how TiDB behaves when the waiting time of SQL optimization reaches the timeout to synchronously load complete column statistics. The default value ON means that the SQL optimization gets back to using pseudo statistics after the timeout. If this variable to OFF, SQL execution fails after the timeout.

tidb_stats_load_sync_wait New in v5.4.0

Note

This variable is read-only for TiDB Serverless.

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Integer
Default value: 100
Range: [0, 2147483647]
Unit: Milliseconds
This variable controls whether to enable the synchronously loading statistics feature. The value 0 means that the feature is disabled. To enable the feature, you can set this variable to a timeout (in milliseconds) that SQL optimization can wait for at most to synchronously load complete column statistics. For details, see Load statistics.

tidb_stmt_summary_enable_persistent New in v6.6.0

Warning

Statements summary persistence is an experimental feature. It is not recommended that you use it in the production environment. This feature might be changed or removed without prior notice. If you find a bug, you can report an issue on GitHub.

Scope: GLOBAL
Applies to hint SET_VAR: No
Type: Boolean
Default value: OFF
This variable is read-only. It controls whether to enable statements summary persistence.

The value of this variable is the same as that of the configuration item tidb_stmt_summary_enable_persistent.

tidb_stmt_summary_filename New in v6.6.0

Warning

Scope: GLOBAL
Applies to hint SET_VAR: No
Type: String
Default value: "tidb-statements.log"
This variable is read-only. It specifies the file to which persistent data is written when statements summary persistence is enabled.

The value of this variable is the same as that of the configuration item tidb_stmt_summary_filename.

tidb_stmt_summary_file_max_backups New in v6.6.0

Warning

Scope: GLOBAL
Applies to hint SET_VAR: No
Type: Integer
Default value: 0
This variable is read-only. It specifies the maximum number of data files that can be persisted when statements summary persistence is enabled.

The value of this variable is the same as that of the configuration item tidb_stmt_summary_file_max_backups.

tidb_stmt_summary_file_max_days New in v6.6.0

Warning

Scope: GLOBAL
Applies to hint SET_VAR: No
Type: Integer
Default value: 3
Unit: day
This variable is read-only. It specifies the maximum number of days to keep persistent data files when statements summary persistence is enabled.

The value of this variable is the same as that of the configuration item tidb_stmt_summary_file_max_days.

tidb_stmt_summary_file_max_size New in v6.6.0

Warning

Scope: GLOBAL
Applies to hint SET_VAR: No
Type: Integer
Default value: 64
Unit: MiB
This variable is read-only. It specifies the maximum size of a persistent data file when statements summary persistence is enabled.

The value of this variable is the same as that of the configuration item tidb_stmt_summary_file_max_size.

tidb_stmt_summary_history_size New in v4.0

Note

This variable is read-only for TiDB Serverless.

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 24
Range: [0, 255]
This variable is used to set the history capacity of statement summary tables.

tidb_stmt_summary_internal_query New in v4.0

Note

This variable is read-only for TiDB Serverless.

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: OFF
This variable is used to control whether to include the SQL information of TiDB in statement summary tables.

tidb_stmt_summary_max_sql_length New in v4.0

Note

This variable is read-only for TiDB Serverless.

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 4096
Range: [0, 2147483647]
Unit: Bytes

This variable is used to control the length of the SQL string in statement summary tables and the TiDB Dashboard.

tidb_stmt_summary_max_stmt_count New in v4.0

Note

This variable is read-only for TiDB Serverless.

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 3000
Range: [1, 32767]
This variable is used to set the maximum number of statements that statement summary tables store in memory.

tidb_stmt_summary_refresh_interval New in v4.0

Note

This variable is read-only for TiDB Serverless.

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 1800
Range: [1, 2147483647]
Unit: Seconds
This variable is used to set the refresh time of statement summary tables.

tidb_store_batch_size

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Integer
Default value: 4
Range: [0, 25000]
This variable is used to control the batch size of the Coprocessor Tasks of the IndexLookUp operator. 0 means to disable batch. When the number of tasks is relatively large and slow queries occur, you can increase this variable to optimize the query.

tidb_store_limit New in v3.0.4 and v4.0

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 0
Range: [0, 9223372036854775807]
This variable is used to limit the maximum number of requests TiDB can send to TiKV at the same time. 0 means no limit.

tidb_streamagg_concurrency

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 1
This variable sets the concurrency of the StreamAgg operator when queries are executed.
It is NOT recommended to set this variable. Modifying the variable value might cause data correctness issues.

tidb_super_read_only New in v5.3.1

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: OFF
tidb_super_read_only aims to be implemented as a replacement of the MySQL variable super_read_only. However, because TiDB is a distributed database, tidb_super_read_only does not make the database read-only immediately after execution, but eventually.
Users with the SUPER or SYSTEM_VARIABLES_ADMIN privilege can modify this variable.
This variable controls the read-only status of the entire cluster. When the variable is ON, all TiDB servers in the entire cluster are in the read-only mode. In this case, TiDB only executes the statements that do not modify data, such as SELECT, USE, and SHOW. For other statements such as INSERT and UPDATE, TiDB rejects executing those statements in the read-only mode.
Enabling the read-only mode using this variable only ensures that the entire cluster finally enters the read-only status. If you have changed the value of this variable in a TiDB cluster but the change has not yet propagated to other TiDB servers, the un-updated TiDB servers are still not in the read-only mode.
TiDB checks the read-only flag before SQL statements are executed. Since v6.2.0, the flag is also checked before SQL statements are committed. This helps prevent the case where long-running auto commit statements might modify data after the server has been placed in read-only mode.
When this variable is enabled, TiDB handles the uncommitted transactions in the following ways:
- For uncommitted read-only transactions, you can commit the transactions normally.
- For uncommitted transactions that are not read-only, SQL statements that perform write operations in these transactions are rejected.
- For uncommitted read-only transactions with modified data, the commit of these transactions is rejected.
After the read-only mode is enabled, all users (including the users with the SUPER privilege) cannot execute the SQL statements that might write data unless the user is explicitly granted the RESTRICTED_REPLICA_WRITER_ADMIN privilege.
When the tidb_restricted_read_only system variable is set to ON, tidb_super_read_only is affected by tidb_restricted_read_only in some cases. For detailed impact, see the description of tidb_restricted_read_only.

tidb_sysdate_is_now New in v6.0.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: OFF
This variable is used to control whether the SYSDATE function can be replaced by the NOW function. This configuration item has the same effect as the MySQL option sysdate-is-now.

tidb_sysproc_scan_concurrency New in v6.5.0

Note

This variable is read-only for TiDB Serverless.

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 1
Range: [0, 4294967295]. The maximum value for v7.5.0 and earlier versions is 256. Before v8.2.0, the minimum value is 1. When you set it to 0, it adaptively adjusts the concurrency based on the cluster size.
This variable is used to set the concurrency of scan operations performed when TiDB executes internal SQL statements (such as an automatic update of statistics).

tidb_table_cache_lease New in v6.0.0

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 3
Range: [1, 10]
Unit: Seconds
This variable is used to control the lease time of cached tables with a default value of 3. The value of this variable affects the modification to cached tables. After a modification is made to cached tables, the longest waiting time might be tidb_table_cache_lease seconds. If the table is read-only or can accept a high write latency, you can increase the value of this variable to increase the valid time for caching tables and to reduce the frequency of lease renewal.

tidb_tmp_table_max_size New in v5.3.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 67108864
Range: [1048576, 137438953472]
Unit: Bytes
This variable is used to set the maximum size of a single temporary table. Any temporary table with a size larger than this variable value causes error.

tidb_top_sql_max_meta_count New in v6.0.0

Note

This TiDB variable is not applicable to TiDB Cloud.

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 5000
Range: [1, 10000]

This variable is used to control the maximum number of SQL statement types collected by Top SQL per minute.

tidb_top_sql_max_time_series_count New in v6.0.0

Note

This TiDB variable is not applicable to TiDB Cloud.

Note

Currently, the Top SQL page in TiDB Dashboard only displays the top 5 types of SQL queries that contribute the most to the load, which is irrelevant with the configuration of tidb_top_sql_max_time_series_count.

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 100
Range: [1, 5000]

This variable is used to control how many SQL statements that contribute the most to the load (that is, top N) can be recorded by Top SQL per minute.

tidb_track_aggregate_memory_usage

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: ON
This variable controls whether TiDB tracks the memory usage of aggregate functions.

Warning

If you disable this variable, TiDB might not accurately track the memory usage and cannot control the memory usage of the corresponding SQL statements.

tidb_tso_client_batch_max_wait_time New in v5.3.0

Note

This variable is read-only for TiDB Serverless.

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Float
Default value: 0
Range: [0, 10]
Unit: Milliseconds
This variable is used to set the maximum waiting time for a batch operation when TiDB requests TSO from PD. The default value is 0, which means no extra waiting time.
When obtaining TSO requests from PD each time, PD Client, used by TiDB, collects as many TSO requests received at the same time as possible. Then, PD Client merges the collected requests in batch into one RPC request and sends the request to PD. This helps reduce the pressure on PD.
After setting this variable to a value greater than 0, TiDB waits for the maximum duration of this value before the end of each batch merge. This is to collect more TSO requests and improve the effect of batch operations.
Scenarios for increasing the value of this variable:
- Due to the high pressure of TSO requests, the CPU of the PD leader reaches a bottleneck, which causes high latency of TSO RPC requests.
- There are not many TiDB instances in the cluster, but every TiDB instance is in high concurrency.
It is recommended to set this variable to a value as small as possible.

Note

Suppose that the TSO RPC latency increases for reasons other than a CPU usage bottleneck of the PD leader (such as network issues). In this case, increasing the value of tidb_tso_client_batch_max_wait_time might increase the execution latency in TiDB and affect the QPS performance of the cluster.

tidb_ttl_delete_rate_limit New in v6.5.0

Note

This variable is read-only for TiDB Serverless.

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Default value: 0
Range: [0, 9223372036854775807]
This variable is used to limit the rate of DELETE statements in TTL jobs on each TiDB node. The value represents the maximum number of DELETE statements allowed per second in a single node in a TTL job. When this variable is set to 0, no limit is applied. For more information, refer to Time to Live.

tidb_ttl_delete_batch_size New in v6.5.0

Note

This variable is read-only for TiDB Serverless.

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Default value: 100
Range: [1, 10240]
This variable is used to set the maximum number of rows that can be deleted in a single DELETE transaction in a TTL job. For more information, refer to Time to Live.

tidb_ttl_delete_worker_count New in v6.5.0

Note

This variable is read-only for TiDB Serverless.

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Default value: 4
Range: [1, 256]
This variable is used to set the maximum concurrency of TTL jobs on each TiDB node. For more information, refer to Time to Live.

tidb_ttl_job_enable New in v6.5.0

Note

This variable is read-only for TiDB Serverless.

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Default value: ON
Type: Boolean
This variable is used to control whether TTL jobs are enabled. If it is set to OFF, all tables with TTL attributes automatically stop cleaning up expired data. For more information, refer to Time to Live.

tidb_ttl_scan_batch_size New in v6.5.0

Note

This variable is read-only for TiDB Serverless.

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Default value: 500
Range: [1, 10240]
This variable is used to set the LIMIT value of each SELECT statement used to scan expired data in a TTL job. For more information, refer to Time to Live.

tidb_ttl_scan_worker_count New in v6.5.0

Note

This variable is read-only for TiDB Serverless.

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Default value: 4
Range: [1, 256]
This variable is used to set the maximum concurrency of TTL scan jobs on each TiDB node. For more information, refer to Time to Live.

tidb_ttl_job_schedule_window_start_time New in v6.5.0

Note

This variable is read-only for TiDB Serverless.

Scope: GLOBAL
Applies to hint SET_VAR: No
Type: Time
Persists to cluster: Yes
Default value: 00:00 +0000
This variable is used to control the start time of the scheduling window of TTL jobs in the background. When you modify the value of this variable, be cautious that a small window might cause the cleanup of expired data to fail. For more information, refer to Time to Live.

tidb_ttl_job_schedule_window_end_time New in v6.5.0

Note

This variable is read-only for TiDB Serverless.

Scope: GLOBAL
Applies to hint SET_VAR: No
Type: Time
Persists to cluster: Yes
Default value: 23:59 +0000
This variable is used to control the end time of the scheduling window of TTL jobs in the background. When you modify the value of this variable, be cautious that a small window might cause the cleanup of expired data to fail. For more information, refer to Time to Live.

tidb_ttl_running_tasks New in v7.0.0

Note

This variable is read-only for TiDB Serverless.

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: -1
Range: -1 and [1, 256]
Specifies the maximum number of running TTL tasks in the entire cluster. -1 means the number of TTL tasks is equivalent to the number of TiKV nodes. For more information, refer to Time to Live.

tidb_txn_assertion_level New in v6.0.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Enumeration
Default value: FAST
Possible values: OFF, FAST, STRICT
This variable is used to control the assertion level. Assertion is a consistency check between data and indexes, which checks whether a key being written exists in the transaction commit process. For more information, see Troubleshoot Inconsistency Between Data and Indexes.
- OFF: Disable this check.
- FAST: Enable most of the check items, with almost no impact on performance.
- STRICT: Enable all check items, with a minor impact on pessimistic transaction performance when the system workload is high.
For new clusters of v6.0.0 or later versions, the default value is FAST. For existing clusters that upgrade from versions earlier than v6.0.0, the default value is OFF.

tidb_txn_commit_batch_size New in v6.2.0

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 16384
Range: [1, 1073741824]
Unit: Bytes

This variable is used to control the batch size of transaction commit requests that TiDB sends to TiKV. If most of the transactions in the application workload have a large number of write operations, adjusting this variable to a larger value can improve the performance of batch processing. However, if this variable is set to too large a value and exceeds the limit of TiKV's raft-entry-max-size, the commits might fail.

tidb_txn_entry_size_limit New in v7.6.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 0
Range: [0, 125829120]
Unit: Bytes

This variable is used to dynamically modify the TiDB configuration item performance.txn-entry-size-limit. It limits the size of a single row of data in TiDB, which is equivalent to the configuration item. The default value of this variable is 0, which means that TiDB uses the value of the configuration item txn-entry-size-limit by default. When this variable is set to a non-zero value, txn-entry-size-limit is also set to the same value.

Note

Modifying this variable with the SESSION scope only affects the current user session, not the internal TiDB session. This might lead to transaction failure if the entry size of an internal TiDB transaction exceeds the limit of the configuration item. Therefore, to dynamically increase the limit, it is recommended that you modify the variable with the GLOBAL scope.

tidb_txn_mode

Note

This variable is read-only for TiDB Serverless.

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Enumeration
Default value: pessimistic
Possible values: pessimistic, optimistic
This variable is used to set the transaction mode. TiDB 3.0 supports the pessimistic transactions. Since TiDB 3.0.8, the pessimistic transaction mode is enabled by default.
If you upgrade TiDB from v3.0.7 or earlier versions to v3.0.8 or later versions, the default transaction mode does not change. Only the newly created clusters use the pessimistic transaction mode by default.
If this variable is set to "optimistic" or "", TiDB uses the optimistic transaction mode.

tidb_use_plan_baselines New in v4.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: ON
This variable is used to control whether to enable the execution plan binding feature. It is enabled by default, and can be disabled by assigning the OFF value. For the use of the execution plan binding, see Execution Plan Binding.

tidb_wait_split_region_finish

Note

This variable is read-only for TiDB Serverless.

Scope: SESSION
Applies to hint SET_VAR: No
Type: Boolean
Default value: ON
It usually takes a long time to scatter Regions, which is determined by PD scheduling and TiKV loads. This variable is used to set whether to return the result to the client after all Regions are scattered completely when the SPLIT REGION statement is being executed:
- ON requires that the SPLIT REGIONS statement waits until all Regions are scattered.
- OFF permits the SPLIT REGIONS statement to return before finishing scattering all Regions.
Note that when scattering Regions, the write and read performances for the Region that is being scattered might be affected. In batch-write or data importing scenarios, it is recommended to import data after Regions scattering is finished.

tidb_wait_split_region_timeout

Note

This variable is read-only for TiDB Serverless.

Scope: SESSION
Applies to hint SET_VAR: No
Type: Integer
Default value: 300
Range: [1, 2147483647]
Unit: Seconds
This variable is used to set the timeout for executing the SPLIT REGION statement. If a statement is not executed completely within the specified time value, a timeout error is returned.

tidb_window_concurrency New in v4.0

Warning

Since v5.0, this variable is deprecated. Instead, use tidb_executor_concurrency for setting.

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: -1
Range: [1, 256]
Unit: Threads
This variable is used to set the concurrency degree of the window operator.
A value of -1 means that the value of tidb_executor_concurrency will be used instead.

tiflash_fastscan New in v6.3.0

Scope: SESSION | GLOBAL
Applies to hint SET_VAR: Yes
Default value: OFF
Type: Boolean
If FastScan is enabled (set to ON), TiFlash provides more efficient query performance, but does not guarantee the accuracy of the query results or data consistency.

tiflash_fine_grained_shuffle_batch_size New in v6.2.0

Scope: SESSION | GLOBAL
Applies to hint SET_VAR: Yes
Default value: 8192
Range: [1, 18446744073709551615]
When Fine Grained Shuffle is enabled, the window function pushed down to TiFlash can be executed in parallel. This variable controls the batch size of the data sent by the sender.
Impact on performance: set a reasonable size according to your business requirements. Improper setting affects the performance. If the value is set too small, for example 1, it causes one network transfer per Block. If the value is set too large, for example, the total number of rows of the table, it causes the receiving end to spend most of the time waiting for data, and the piplelined computation cannot work. To set a proper value, you can observe the distribution of the number of rows received by the TiFlash receiver. If most threads receive only a few rows, for example a few hundred, you can increase this value to reduce the network overhead.

tiflash_fine_grained_shuffle_stream_count New in v6.2.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Integer
Default value: 0
Range: [-1, 1024]
When the window function is pushed down to TiFlash for execution, you can use this variable to control the concurrency level of the window function execution. The possible values are as follows:
- -1: the Fine Grained Shuffle feature is disabled. The window function pushed down to TiFlash is executed in a single thread.
- 0: the Fine Grained Shuffle feature is enabled. If tidb_max_tiflash_threads is set to a valid value (greater than 0), then tiflash_fine_grained_shuffle_stream_count is set to the value of tidb_max_tiflash_threads. Otherwise, it is automatically estimated based on the CPU resources of the TiFlash compute node. The actual concurrency level of the window function on TiFlash is: min(tiflash_fine_grained_shuffle_stream_count, the number of physical threads on TiFlash nodes).
- Integer greater than 0: the Fine Grained Shuffle feature is enabled. The window function pushed down to TiFlash is executed in multiple threads. The concurrency level is: min(tiflash_fine_grained_shuffle_stream_count, the number of physical threads on TiFlash nodes).
Theoretically, the performance of the window function increases linearly with this value. However, if the value exceeds the actual number of physical threads, it instead leads to performance degradation.

tiflash_mem_quota_query_per_node New in v7.4.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 0
Range: [-1, 9223372036854775807]
This variable limits the maximum memory usage for a query on a TiFlash node. When the memory usage of a query exceeds this limit, TiFlash returns an error and terminates the query. Setting this variable to -1 or 0 means no limit. When this variable is set to a value greater than 0 and tiflash_query_spill_ratio is set to a valid value, TiFlash enables query-level spilling.

tiflash_query_spill_ratio New in v7.4.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Float
Default value: 0.7
Range: [0, 0.85]
This variable controls the threshold for TiFlash query-level spilling. 0 means disabling the automatic query-level spilling. When this variable is greater than 0 and the memory usage of a query exceeds tiflash_mem_quota_query_per_node * tiflash_query_spill_ratio, TiFlash triggers query-level spilling, which spills data of supported operators in the query as needed.

Note

This variable only takes effect when tiflash_mem_quota_query_per_node is greater than 0. In other words, if tiflash_mem_quota_query_per_node is 0 or -1, query-level spilling will not be enabled even if tiflash_query_spill_ratio is greater than 0.
When TiFlash query-level spilling is enabled, the spilling thresholds for individual TiFlash operators automatically become invalidated. In other words, if both tiflash_mem_quota_query_per_node and tiflash_query_spill_ratio are greater than 0, the three variables tidb_max_bytes_before_tiflash_external_sort, tidb_max_bytes_before_tiflash_external_group_by, and tidb_max_bytes_before_tiflash_external_join become invalidated automatically, equivalent to setting them to 0.

tiflash_replica_read New in v7.3.0

Note

This TiDB variable is not applicable to TiDB Cloud.

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Enumeration
Default value: all_replicas
Value options: all_replicas, closest_adaptive, or closest_replicas
This variable is used to set the strategy for selecting TiFlash replicas when a query requires the TiFlash engine.
- all_replicas means using all available TiFlash replicas for analytical computing.
- closest_adaptive means preferring to use TiFlash replicas in the same zone as the TiDB node initiating the query. If replicas in this zone do not contain all the required data, the query will involve TiFlash replicas from other zones along with their corresponding TiFlash nodes.
- closest_replicas means using only TiFlash replicas in the same zone as the TiDB node initiating the query. If replicas in this zone do not contain all the required data, the query will return an error.

Note

If TiDB nodes do not have zone attributes configured and tiflash_replica_read is not set to all_replicas, TiFlash ignores the replica selection strategy. Instead, it uses all TiFlash replicas for queries and returns the The variable tiflash_replica_read is ignored. warning.
If TiFlash nodes do not have zone attributes configured, they are treated as nodes not belonging to any zone.

tiflash_hashagg_preaggregation_mode New in v8.3.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: Yes
Type: Enumeration
Default value: force_preagg
Value options: force_preagg, force_streaming, auto
This variable controls the pre-aggregation strategy used during the first stage of two-stage or three-stage HashAgg operations pushed down to TiFlash:
- force_preagg: TiFlash forces pre-aggregation during the first stage of HashAgg. This behavior is consistent with the behavior before v8.3.0.
- force_streaming: TiFlash directly sends data to the next stage of HashAgg without pre-aggregation.
- auto: TiFlash automatically chooses whether to perform pre-aggregation based on the current workload's aggregation degree.

tikv_client_read_timeout New in v7.4.0

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 0
Range: [0, 2147483647]
Unit: Millisecond
You can use tikv_client_read_timeout to set the timeout for TiDB to send a TiKV RPC read request in a query. When a TiDB cluster is in an environment with unstable network or serious TiKV I/O latency jitter, and your application is sensitive to the latency of the SQL queries, you can set tikv_client_read_timeout to reduce the timeout of the TiKV RPC read requests. In this case, when a TiKV node has I/O latency jitter, TiDB can time out quickly and re-send the RPC request to the TiKV node where the next TiKV Region Peer is located. If the requests of all TiKV Region Peers time out, TiDB will retry with the default timeout (usually 40 seconds).
You can also use the optimizer hint /*+ SET_VAR(TIKV_CLIENT_READ_TIMEOUT=N) */ in a query to set the timeout for TiDB to send a TiKV RPC read request. If both the optimizer hint and this system variable are set, the optimizer hint takes higher priority.
The default value 0 indicates that the default timeout (usually 40 seconds) is used.

Note

Normally, a regular query takes a few milliseconds, but occasionally when a TiKV node is in unstable network or gets I/O jitter, the query can take more than 1 second or even 10 seconds. In this case, you can set the TiKV RPC read request timeout to 100 milliseconds for a specific query by using the optimizer hint /*+ SET_VAR(TIKV_CLIENT_READ_TIMEOUT=100) */. In this way, even if the response of a TiKV node is slow, TiDB can quickly time out and then re-send the RPC request to the TiKV node where the next TiKV Region Peer is located. Because the probability of two TiKV nodes getting I/O jitter simultaneously is low, the query can be completed usually within a few milliseconds to 110 milliseconds.
Do not set too small values (for example, 1 millisecond) for tikv_client_read_timeout. Otherwise, the requests might time out easily when the workload of a TiDB cluster is high, and subsequent retries will further increase the load on the TiDB cluster.
If you need to set different timeout values for different types of queries, it is recommended to use optimizer hints.

time_zone

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Default value: SYSTEM
This variable returns the current time zone. Values can be specified as either an offset such as '-8:00' or a named zone 'America/Los_Angeles'.
The value SYSTEM means that the time zone should be the same as the system host, which is available via the system_time_zone variable.

timestamp

Scope: SESSION
Applies to hint SET_VAR: No
Type: Float
Default value: 0
Range: [0, 2147483647]
A non-empty value of this variable indicates the UNIX epoch that is used as the timestamp for CURRENT_TIMESTAMP(), NOW(), and other functions. This variable might be used in data restore or replication.

transaction_isolation

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Enumeration
Default value: REPEATABLE-READ
Possible values: READ-UNCOMMITTED, READ-COMMITTED, REPEATABLE-READ, SERIALIZABLE
This variable sets the transaction isolation. TiDB advertises REPEATABLE-READ for compatibility with MySQL, but the actual isolation level is Snapshot Isolation. See transaction isolation levels for further details.

tx_isolation

This variable is an alias for transaction_isolation.

tx_isolation_one_shot

Note

This variable is internally used in TiDB. You are not expected to use it.

Internally, the TiDB parser transforms the SET TRANSACTION ISOLATION LEVEL [READ COMMITTED| REPEATABLE READ | ...] statements to SET @@SESSION.TX_ISOLATION_ONE_SHOT = [READ COMMITTED| REPEATABLE READ | ...].

tx_read_ts

Scope: SESSION
Applies to hint SET_VAR: No
Default value: ""
In the Stale Read scenarios, this session variable is used to help record the Stable Read timestamp value.
This variable is used for the internal operation of TiDB. It is NOT recommended to set this variable.

txn_scope

Note

This variable is read-only for TiDB Serverless.

Scope: SESSION
Applies to hint SET_VAR: No
Default value: global
Value options: global and local
This variable is used to set whether the current session transaction is a global transaction or a local transaction.
This variable is used for the internal operation of TiDB. It is NOT recommended to set this variable.

validate_password.check_user_name New in v6.5.0

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Default value: ON
Type: Boolean
This variable is a check item in the password complexity check. It checks whether the password matches the username. This variable takes effect only when validate_password.enable is enabled.
When this variable is effective and set to ON, if you set a password, TiDB compares the password with the username (excluding the hostname). If the password matches the username, the password is rejected.
This variable is independent of validate_password.policy and not affected by the password complexity check level.

validate_password.dictionary New in v6.5.0

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Default value: ""
Type: String
This variable is a check item in the password complexity check. It checks whether the password matches the dictionary. This variable takes effect only when validate_password.enable is enabled and validate_password.policy is set to 2 (STRONG).
This variable is a string not longer than 1024 characters. It contains a list of words that cannot exist in the password. Each word is separated by semicolon (;).
This variable is set to an empty string by default, which means no dictionary check is performed. To perform the dictionary check, you need to include the words to be matched in the string. If this variable is configured, when you set a password, TiDB compares each substring (length in 4 to 100 characters) of the password with the words in the dictionary. If any substring of the password matches a word in the dictionary, the password is rejected. The comparison is case-insensitive.

validate_password.enable New in v6.5.0

Note

This variable is always enabled for TiDB Serverless.

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Default value: OFF
Type: Boolean
This variable controls whether to perform password complexity check. If this variable is set to ON, TiDB performs the password complexity check when you set a password.

validate_password.length New in v6.5.0

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 8
Range: [0, 2147483647] for TiDB Self-Hosted and TiDB Dedicated, [8, 2147483647] for TiDB Serverless
This variable is a check item in the password complexity check. It checks whether the password length is sufficient. By default, the minimum password length is 8. This variable takes effect only when validate_password.enable is enabled.
The value of this variable must not be smaller than the expression: validate_password.number_count + validate_password.special_char_count + (2 * validate_password.mixed_case_count).
If you change the value of validate_password.number_count, validate_password.special_char_count, or validate_password.mixed_case_count such that the expression value is larger than validate_password.length, the value of validate_password.length is automatically changed to match the expression value.

validate_password.mixed_case_count New in v6.5.0

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 1
Range: [0, 2147483647] for TiDB Self-Hosted and TiDB Dedicated, [1, 2147483647] for TiDB Serverless
This variable is a check item in the password complexity check. It checks whether the password contains sufficient uppercase and lowercase letters. This variable takes effect only when validate_password.enable is enabled and validate_password.policy is set to 1 (MEDIUM) or larger.
Neither the number of uppercase letters nor the number of lowercase letters in the password can be fewer than the value of validate_password.mixed_case_count. For example, when the variable is set to 1, the password must contain at least one uppercase letter and one lowercase letter.

validate_password.number_count New in v6.5.0

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 1
Range: [0, 2147483647] for TiDB Self-Hosted and TiDB Dedicated, [1, 2147483647] for TiDB Serverless
This variable is a check item in the password complexity check. It checks whether the password contains sufficient numbers. This variable takes effect only when validate_password.enable is enabled and validate_password.policy is set to 1 (MEDIUM) or larger.

validate_password.policy New in v6.5.0

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Enumeration
Default value: 1
Value options: 0, 1, and 2 for TiDB Self-Hosted and TiDB Dedicated; 1 and 2 for TiDB Serverless
This variable controls the policy for the password complexity check. This variable takes effect only when validate_password.enable is enabled. The value of this variable determines whether other validate-password variables take effect in the password complexity check, except for validate_password.check_user_name.
This value of this variable can be 0, 1, or 2 (corresponds to LOW, MEDIUM, or STRONG). Different policy levels have different checks:
- 0 or LOW: password length.
- 1 or MEDIUM: password length, uppercase and lowercase letters, numbers, and special characters.
- 2 or STRONG: password length, uppercase and lowercase letters, numbers, special characters, and dictionary match.

validate_password.special_char_count New in v6.5.0

Scope: GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 1
Range: [0, 2147483647] for TiDB Self-Hosted and TiDB Dedicated, [1, 2147483647] for TiDB Serverless
This variable is a check item in the password complexity check. It checks whether the password contains sufficient special characters. This variable takes effect only when validate_password.enable is enabled and validate_password.policy is set to 1 (MEDIUM) or larger.

version

Scope: NONE
Applies to hint SET_VAR: No
Default value: 8.0.11-TiDB-(tidb version)
This variable returns the MySQL version, followed by the TiDB version. For example '8.0.11-TiDB-v8.3.0'.

version_comment

Scope: NONE
Applies to hint SET_VAR: No
Default value: (string)
This variable returns additional details about the TiDB version. For example, 'TiDB Server (Apache License 2.0) Community Edition, MySQL 8.0 compatible'.

version_compile_machine

Scope: NONE
Applies to hint SET_VAR: No
Default value: (string)
This variable returns the name of the CPU architecture on which TiDB is running.

version_compile_os

Scope: NONE
Applies to hint SET_VAR: No
Default value: (string)
This variable returns the name of the OS on which TiDB is running.

wait_timeout

Note

This variable is read-only for TiDB Serverless.

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Integer
Default value: 28800
Range: [0, 31536000]
Unit: Seconds
This variable controls the idle timeout of user sessions. A zero-value means unlimited.

warning_count

Scope: SESSION
Applies to hint SET_VAR: No
Default value: 0
This read-only variable indicates the number of warnings that occurred in the statement that was previously executed.

windowing_use_high_precision

Scope: SESSION | GLOBAL
Persists to cluster: Yes
Applies to hint SET_VAR: No
Type: Boolean
Default value: ON
This variable controls whether to use the high precision mode when computing the window functions.

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