Sign InTry Free

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;

Variable Reference

allow_auto_random_explicit_insert New in v4.0.3

  • Scope: SESSION | GLOBAL
  • Persists to cluster: Yes
  • 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.

auto_increment_increment

  • Scope: SESSION | GLOBAL
  • Persists to cluster: Yes
  • Type: Integer
  • Default value: 1
  • Range: [1, 65535]
  • Controls the step size of AUTO_INCREMENT values to be allocated to a column. It is often used in combination with auto_increment_offset.

auto_increment_offset

  • Scope: SESSION | GLOBAL
  • Persists to cluster: Yes
  • Type: Integer
  • Default value: 1
  • Range: [1, 65535]
  • Controls the initial offset of AUTO_INCREMENT values to be allocated to a column. 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
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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
  • Default value: utf8mb4
  • The default character set for the server.

collation_connection

  • Scope: SESSION | GLOBAL
  • Persists to cluster: Yes
  • 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
  • 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
  • Default value: utf8mb4_bin
  • The default collation used when the database is created.

cte_max_recursion_depth

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

datadir

  • Scope: NONE
  • Default value: /tmp/tidb
  • This variable indicates the location where data is stored. This location can be a local path or point to a PD server if the data is stored on TiKV.
  • A value in the format of ip_address: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.
  • 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
  • Type: Enumeration
  • Default value: mysql_native_password
  • Possible values: mysql_native_password, caching_sha2_password, tidb_sm3_password, and tidb_auth_token
  • The tidb_auth_token authentication method is used only for the internal operation of TiDB Cloud. DO NOT set the variable to this value.
  • 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_password_lifetime New in v6.5.0

  • Scope: GLOBAL
  • Persists to cluster: Yes
  • 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
  • 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
  • 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 default behavior of the client connection for the expired password, contact TiDB Cloud Support.

error_count

  • Scope: SESSION
  • 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
  • Type: Boolean
  • Default value: OFF
  • For compatibility, TiDB returns foreign key checks as OFF.

group_concat_max_len

  • Scope: SESSION | GLOBAL
  • Persists to cluster: Yes
  • Type: Integer
  • Default value: 1024
  • Range: [4, 18446744073709551615]
  • The maximum buffer size for items in the GROUP_CONCAT() function.

have_openssl

  • Scope: NONE
  • 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
  • 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
  • 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
  • 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
  • Type: Integer
  • Default value: 50
  • Range: [1, 3600]
  • Unit: Seconds
  • The lock wait timeout for pessimistic transactions (default).

interactive_timeout

  • Scope: SESSION | GLOBAL
  • Persists to cluster: Yes
  • 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
  • Type: Integer
  • Default value: 0
  • Range: [0, 9223372036854775807]
  • 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
  • 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
  • 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
  • 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
  • Default value: Apache License 2.0
  • This variable indicates the license of your TiDB server installation.

log_bin

  • Scope: NONE
  • Type: Boolean
  • Default value: OFF
  • This variable indicates whether TiDB Binlog is used.

max_allowed_packet New in v6.1.0

  • Scope: SESSION | GLOBAL
  • Persists to cluster: Yes
  • 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.
  • This variable is compatible with MySQL.

password_history New in v6.5.0

  • Scope: GLOBAL
  • Persists to cluster: Yes
  • 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.

password_reuse_interval New in v6.5.0

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

max_prepared_stmt_count

  • Scope: GLOBAL
  • Persists to cluster: Yes
  • 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)

plugin_dir

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

plugin_load

  • Scope: GLOBAL
  • Persists to cluster: No, only applicable to the current TiDB instance that you are connecting to.
  • 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
  • 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
  • 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
  • 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

  • Scope: GLOBAL
  • Persists to cluster: Yes
  • Type: Boolean
  • Default value: OFF
  • This variable ensures that all connections to TiDB are either on a local socket, or using TLS.
  • 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.

skip_name_resolve New in v5.2.0

  • Scope: GLOBAL
  • Persists to cluster: Yes
  • 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.

socket

  • Scope: NONE
  • Default value: ""
  • The local unix socket file that the tidb-server is listening on when speaking the MySQL protocol.

sql_log_bin

  • Scope: SESSION | GLOBAL
  • Persists to cluster: Yes
  • Type: Boolean
  • Default value: ON
  • Indicates whether to write changes to TiDB Binlog or not.

sql_mode

  • Scope: SESSION | GLOBAL
  • Persists to cluster: 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
  • 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.

sql_select_limit New in v4.0.2

  • Scope: SESSION | GLOBAL
  • Persists to cluster: Yes
  • Type: Integer
  • Default value: 18446744073709551615
  • Range: [0, 18446744073709551615]
  • Unit: Rows
  • The maximum number of rows returned by the SELECT statements.

ssl_ca

  • Scope: NONE
  • 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.
  • Scope: NONE
  • 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
  • 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.
  • Scope: NONE
  • 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
  • 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.
  • Scope: NONE
  • 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
  • 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
  • Default value: 4096
  • Range: [0, 9223372036854775807]
  • 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_batch_cop New in v4.0

  • Scope: SESSION | GLOBAL

  • Persists to cluster: 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
  • 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
  • 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 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
  • 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.
    • 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
  • 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_partition_concurrency

  • Scope: SESSION | GLOBAL
  • Persists to cluster: Yes
  • Default value: 1
  • 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
  • Type: Integer
  • Default value: 2
  • Default value: 1
  • Range: [1, 2]
  • Controls how TiDB collects statistics.
  • In v5.3.0 and later versions, the default value of this variable is 2. If your cluster is upgraded from a version earlier than v5.3.0 to v5.3.0 or later, the default value of tidb_analyze_version does not change. For detailed introduction, see Introduction to Statistics.

tidb_auto_analyze_end_time

  • Scope: GLOBAL
  • Persists to cluster: Yes
  • 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 1AM and 3AM, 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
  • Default value: 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
  • Type: Float
  • Default value: 0.5
  • Range: [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.

tidb_auto_analyze_start_time

  • Scope: GLOBAL
  • Persists to cluster: Yes
  • 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, 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
  • 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
  • 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

  • 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

  • Scope: SESSION
  • 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

  • Scope: SESSION
  • 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

  • Scope: SESSION
  • 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
  • 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
  • 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.

tidb_broadcast_join_threshold_size New in v5.0

  • Scope: SESSION | GLOBAL
  • Persists to cluster: 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.

tidb_build_stats_concurrency

  • Scope: SESSION | GLOBAL
  • Persists to cluster: Yes
  • Type: Integer
  • Default value: 4
  • 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_capture_plan_baselines New in v4.0

  • Scope: GLOBAL
  • Persists to cluster: Yes
  • 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

  • Scope: SESSION
  • Persists to cluster: 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

  • Scope: GLOBAL
  • Persists to cluster: No, only applicable to the current TiDB instance that you are connecting to.
  • 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
  • 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
  • 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

  • Scope: GLOBAL
  • Persists to cluster: No, only applicable to the current TiDB instance that you are connecting to.
  • 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

  • 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
  • Type: Boolean
  • Default value: ON
  • 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

  • Scope: SESSION | GLOBAL
  • Persists to cluster: 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
  • 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

  • Scope: GLOBAL
  • Persists to cluster: Yes
  • 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

  • Scope: GLOBAL
  • Persists to cluster: Yes
  • 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.
  • 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.

tidb_ddl_error_count_limit

  • Scope: GLOBAL
  • Persists to cluster: Yes
  • 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

  • Scope: GLOBAL
  • Persists to cluster: Yes
  • Type: Integer
  • Default value: 64
  • Range: [1, 256]
  • This variable controls the concurrency of FLASHBACK CLUSTER TO TIMESTAMP.

tidb_ddl_reorg_batch_size

  • Scope: GLOBAL
  • Persists to cluster: Yes
  • 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 many updating operations such as UPDATE and REPLACE exist during the ADD INDEX operation, a larger batch size indicates a larger probability of transaction conflicts. In this case, you need to adjust the batch size to a smaller value. The minimum value is 32.
    • If the transaction conflict does not exist, you can set the batch size to a large value (consider the worker count. See Interaction Test on Online Workloads and ADD INDEX Operations for reference). This can increase the speed of the backfilling data, but the write pressure on TiKV also becomes higher.

tidb_ddl_reorg_priority

  • Scope: SESSION
  • 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

  • Scope: GLOBAL
  • Persists to cluster: Yes
  • 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.

tidb_default_string_match_selectivity New in v6.2.0

  • Scope: SESSION | GLOBAL
  • Persists to cluster: 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

  • Scope: SESSION | GLOBAL

  • Persists to cluster: Yes

  • 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.

    This variable only applies to optimistic transactions, not to pessimistic transactions. The number of retries for pessimistic transactions is 256.

tidb_distsql_scan_concurrency

  • Scope: SESSION | GLOBAL
  • Persists to cluster: 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).

tidb_dml_batch_size

  • Scope: SESSION | GLOBAL
  • Persists to cluster: Yes
  • 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 or LOAD DATA 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.

tidb_enable_1pc New in v5.0

  • Scope: SESSION | GLOBAL
  • Persists to cluster: Yes
  • 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.

tidb_enable_amend_pessimistic_txn New in v4.0.7

  • Scope: SESSION | GLOBAL

  • Persists to cluster: Yes

  • Type: Boolean

  • Default value: OFF

  • This variable is used to control whether to enable the AMEND TRANSACTION feature. If you enable the AMEND TRANSACTION feature in a pessimistic transaction, when concurrent DDL operations and SCHEMA VERSION changes exist on tables associated with this transaction, TiDB attempts to amend the transaction. TiDB corrects the transaction commit to make the commit consistent with the latest valid SCHEMA VERSION so that the transaction can be successfully committed without getting the Information schema is changed error. This feature is effective on the following concurrent DDL operations:

    • ADD COLUMN or DROP COLUMN operations.
    • MODIFY COLUMN or CHANGE COLUMN operations which increase the length of a field.
    • ADD INDEX or DROP INDEX operations in which the index column is created before the transaction is opened.

tidb_enable_analyze_snapshot New in v6.2.0

  • Scope: SESSION | GLOBAL
  • Persists to cluster: Yes
  • 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.

tidb_enable_async_commit New in v5.0

  • Scope: SESSION | GLOBAL
  • Persists to cluster: Yes
  • 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.

tidb_enable_auto_analyze New in v6.1.0

  • Scope: GLOBAL
  • Persists to cluster: Yes
  • 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_increment_in_generated

  • Scope: SESSION | GLOBAL
  • Persists to cluster: Yes
  • 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

  • Scope: GLOBAL
  • Persists to cluster: Yes
  • 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

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

tidb_enable_chunk_rpc New in v4.0

  • Scope: SESSION
  • 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
  • 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

  • Scope: GLOBAL
  • Persists to cluster: No, only applicable to the current TiDB instance that you are connecting to.
  • Default value: ON
  • Possible values: OFF, ON
  • This variable controls whether the corresponding TiDB server can run DDL statements or not.

tidb_enable_collect_execution_info

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

tidb_enable_column_tracking New in v5.4.0

  • Scope: GLOBAL
  • Persists to cluster: Yes
  • 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_concurrent_ddl New in v6.2.0

  • Scope: GLOBAL
  • Persists to cluster: Yes
  • Type: Boolean
  • Default value: ON
  • This variable controls whether to allow TiDB to use concurrent DDL statements. When concurrent DDL statements are used, the DDL execution flow is changed, and DDL statements are not easily blocked by other DDL statements. In addition, multiple indexes can be added at the same time.

tidb_enable_enhanced_security

  • Scope: NONE
  • 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.
  • Default value: ON
  • This variable is read-only. For TiDB Cloud, the Security Enhanced Mode (SEM) is enabled by default.
  • 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
  • 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
  • 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
  • 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
  • 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

  • Scope: SESSION | GLOBAL
  • Persists to cluster: Yes
  • 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_foreign_key New in v6.3.0

  • Scope: GLOBAL
  • Persists to cluster: Yes
  • Type: Boolean
  • Default value: OFF
  • This variable controls whether to enable the FOREIGN KEY feature.

tidb_enable_gc_aware_memory_track

  • Scope: GLOBAL
  • Persists to cluster: Yes
  • Type: Boolean
  • Default value: OFF
  • This variable controls whether to enable GC-Aware memory track.

tidb_enable_general_plan_cache New in v6.3.0

  • Scope: SESSION | GLOBAL
  • Persists to cluster: Yes
  • Type: Boolean
  • Default value: OFF
  • This variable controls whether to enable the General Plan Cache feature.

tidb_enable_gogc_tuner New in v6.4.0

  • Scope: GLOBAL
  • Persists to cluster: No, only applicable to the current TiDB instance that you are connecting to.
  • Type: Boolean
  • Default value: ON
  • This variable controls whether to enable GOGC Tuner.

tidb_enable_historical_stats

  • Scope: GLOBAL
  • Persists to cluster: Yes
  • Type: Boolean
  • Default value: OFF
  • This variable is used for an unreleased feature. Do not change the variable value.

tidb_enable_index_merge New in v4.0

  • Scope: SESSION | GLOBAL
  • Persists to cluster: 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
  • 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
  • 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
  • Type: Boolean
  • Default value: ON
  • This variable is used to set whether to enable the LIST (COLUMNS) TABLE PARTITION feature.

tidb_enable_local_txn

  • Scope: GLOBAL
  • Persists to cluster: Yes
  • 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
  • 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
  • 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
  • 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
  • Type: Boolean
  • Default value: OFF
  • This variable controls the behavior when TiDB performs the ONLY_FULL_GOUP_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
  • 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

tidb_enable_noop_variables New in v6.2.0

  • Scope: GLOBAL
  • Persists to cluster: Yes
  • 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
  • Default value: OFF
  • 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.

tidb_enable_outer_join_reorder New in v6.1.0

  • Scope: SESSION | GLOBAL

  • Persists to cluster: 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_ordered_result_mode

  • Scope: SESSION | GLOBAL
  • Persists to cluster: 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

  • 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.

tidb_enable_parallel_apply New in v5.0

  • Scope: SESSION | GLOBAL
  • Persists to cluster: Yes
  • 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_pipelined_window_function

  • Scope: SESSION | GLOBAL
  • Persists to cluster: Yes
  • Type: Boolean
  • Default value: ON
  • This variables specifies whether to use the pipeline execution algorithm for window functions.

tidb_enable_plan_replayer_capture

Warning:

This variable controls a feature that is not fully functional in the current TiDB version. Do not change the default value.

  • Scope: SESSION | GLOBAL
  • Persists to cluster: Yes
  • Type: Boolean
  • Default value: OFF

tidb_enable_prepared_plan_cache New in v6.1.0

  • Scope: SESSION | GLOBAL
  • Persists to cluster: 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
  • 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
  • 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.
  • 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.
  • 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
  • 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.
  • When the operator that reads data has only one thread left and the memory usage of a single SQL statement continues to exceed tidb_mem_quota_query, this SQL statement triggers other memory control behaviors, such as spilling data to disk.

tidb_enable_reuse_chunk New in v6.4.0

  • Scope: SESSION | GLOBAL
  • Persists to cluster: Yes
  • 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_slow_log

  • Scope: GLOBAL
  • Persists to cluster: No, only applicable to the current TiDB instance that you are connecting to.
  • 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
  • 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

  • Scope: GLOBAL
  • Persists to cluster: Yes
  • 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
  • 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)

tidb_enable_table_partition

  • Scope: SESSION | GLOBAL
  • Persists to cluster: Yes
  • 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

  • Scope: GLOBAL
  • Persists to cluster: Yes
  • Type: Boolean
  • Default value: ON
  • This variable is used to dynamically control whether the telemetry collection in TiDB is enabled. By setting the value to OFF, the telemetry collection is disabled. If the enable-telemetry TiDB configuration item is set to false on all TiDB instances, the telemetry collection is always disabled and this system variable will not take effect. See Telemetry for details.
  • This variable is used to dynamically control whether the telemetry collection in TiDB is enabled.

tidb_enable_tiflash_read_for_write_stmt New in v6.3.0

  • Scope: SESSION | GLOBAL

  • Persists to cluster: Yes

  • Type: Boolean

  • Default value: OFF

  • This variable controls whether read operations in SQL statements containing INSERT, DELETE, and UPDATE can be pushed down to TiFlash. For example:

tidb_enable_top_sql New in v5.4.0

  • Scope: GLOBAL
  • Persists to cluster: Yes
  • Type: Boolean
  • Default value: OFF
  • This variable is used to control whether to enable the Top SQL feature.
  • This variable is used to control whether to enable the Top SQL feature.

tidb_enable_tso_follower_proxy New in v5.3.0

  • Scope: GLOBAL
  • Persists to cluster: Yes
  • Type: Boolean
  • Default value: OFF
  • This variable is used to enable the TSO Follower Proxy feature. When the value is OFF, TiDB only gets TSO from the PD leader. After this feature is enabled, TiDB gets TSO by evenly sending requests to all PD nodes and forwarding TSO requests through PD followers. This helps reduce the CPU pressure of 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.

tidb_enable_unsafe_substitute New in v6.3.0

  • Scope: SESSION | GLOBAL
  • Persists to cluster: Yes
  • 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
  • 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
  • Type: Boolean
  • Default value: ON
  • This variable is used to control whether to enable the support for window functions. Note that window functions may use reserved keywords. This might cause SQL statements that could be executed normally cannot be parsed after upgrading TiDB. In this case, you can set tidb_enable_window_function to OFF.

tidb_enforce_mpp New in v5.1

  • Scope: SESSION
  • Type: Boolean
  • Default value: OFF
  • 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.

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_evolve_plan_baselines New in v4.0

  • Scope: SESSION | GLOBAL
  • Persists to cluster: Yes
  • 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
  • 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
  • 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
  • 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
  • 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

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

  • Scope: GLOBAL
  • Persists to cluster: No, only applicable to the current TiDB instance that you are connecting to.
  • 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_force_priority

  • Scope: GLOBAL
  • Persists to cluster: No, only applicable to the current TiDB instance that you are connecting to.
  • 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.

tidb_gc_concurrency New in v5.0

  • Scope: GLOBAL
  • Persists to cluster: Yes
  • Type: Integer
  • Default value: -1
  • Range: [1, 256]
  • Unit: Threads
  • Specifies the number of threads in the Resolve Locks step of GC. A value of -1 means that TiDB will automatically decide the number of garbage collection threads to use.

tidb_gc_enable New in v5.0

  • Scope: GLOBAL
  • Persists to cluster: Yes
  • 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

  • Scope: GLOBAL
  • Persists to cluster: Yes
  • 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.

tidb_gc_max_wait_time New in v6.1.0

  • Scope: GLOBAL
  • Persists to cluster: Yes
  • 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

  • Scope: GLOBAL
  • Persists to cluster: Yes
  • 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

  • Scope: GLOBAL
  • Persists to cluster: Yes
  • 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.
  • 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, thus improving the execution speed in the Resolve Locks step.

tidb_general_log

  • Scope: GLOBAL
  • Persists to cluster: No, only applicable to the current TiDB instance that you are connecting to.
  • 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 you need to trace all SQL statements when locating issues, enable this feature.
  • 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_general_plan_cache_size New in v6.3.0

  • Scope: SESSION | GLOBAL
  • Persists to cluster: Yes
  • Type: Integer
  • Default value: 100
  • Range: [1, 100000]
  • This variable controls the maximum number of execution plans that can be cached by General Plan Cache.

tidb_generate_binary_plan New in v6.2.0

  • Scope: GLOBAL
  • Persists to cluster: Yes
  • 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_threshold New in v6.4.0

  • Scope: GLOBAL
  • Persists to cluster: No, only applicable to the current TiDB instance that you are connecting to.
  • 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

  • Scope: SESSION | GLOBAL
  • Persists to cluster: Yes
  • 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
  • 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

  • Scope: SESSION | GLOBAL
  • Persists to cluster: Yes
  • 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

  • Scope: SESSION | GLOBAL
  • Persists to cluster: Yes
  • 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

  • Scope: SESSION | GLOBAL
  • Persists to cluster: Yes
  • 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_ignore_prepared_cache_close_stmt New in v6.0.0

tidb_index_join_batch_size

  • Scope: SESSION | GLOBAL
  • Persists to cluster: 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_lookup_concurrency

  • Scope: SESSION | GLOBAL
  • Persists to cluster: Yes
  • 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

  • Scope: SESSION | GLOBAL
  • Persists to cluster: Yes
  • 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
  • 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
  • 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
  • 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
  • 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.

tidb_isolation_read_engines New in v4.0

  • Scope: SESSION
  • 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
  • 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
  • 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.

tidb_last_txn_info New in v4.0.9

  • Scope: SESSION
  • 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
  • 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_log_file_max_days New in v5.3.0

  • Scope: GLOBAL
  • Persists to cluster: No, only applicable to the current TiDB instance that you are connecting to.
  • 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.
  • This variable is used to set the maximum days that the log is retained on the current TiDB instance.

tidb_low_resolution_tso

  • Scope: SESSION
  • Type: Boolean
  • Default value: OFF
  • This variable is used to set whether to enable the low precision TSO feature. After this feature is enabled, new transactions use a timestamp updated every 2 seconds to read data.
  • The main applicable scenario is to reduce the overhead of acquiring TSO for small read-only transactions when reading old data is acceptable.

tidb_max_auto_analyze_time New in v6.1.0

  • Scope: GLOBAL
  • Persists to cluster: Yes
  • 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_chunk_size

  • Scope: SESSION | GLOBAL
  • Persists to cluster: Yes
  • 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.

tidb_max_delta_schema_count New in v2.1.18 and v3.0.5

  • Scope: GLOBAL
  • Persists to cluster: Yes
  • 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
  • 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
  • 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. 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
  • 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.
  • 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.
  • 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

  • Scope: GLOBAL
  • Persists to cluster: Yes
  • 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.

tidb_mem_quota_apply_cache New in v5.0

  • Scope: SESSION | GLOBAL
  • Persists to cluster: Yes
  • 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
  • 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
  • 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.
  • For versions earlier than TiDB v6.1.0, this is a session scope variable. 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.
  • 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
  • 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
  • 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

  • Scope: GLOBAL
  • Persists to cluster: Yes
  • 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.
  • This variable sets the memory usage ratio that triggers the tidb-server memory alarm.
  • 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.

tidb_memory_usage_alarm_keep_record_num New in v6.4.0

  • Scope: GLOBAL
  • Persists to cluster: Yes
  • 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
  • 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
  • Default value: 1
  • This variable specifies the concurrency of merging statistics for a partitioned table when TiDB analyzes the partitioned table.

tidb_metric_query_range_duration New in v4.0

  • Scope: SESSION
  • 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

  • Scope: SESSION
  • 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
  • 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
  • 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
  • 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 settingtidb_multi_statement_mode valueMultiple statements permitted?
Multiple Statements = ONOFFYes
Multiple Statements = ONONYes
Multiple Statements = ONWARNYes
Multiple Statements = OFFOFFNo
Multiple Statements = OFFONYes
Multiple Statements = OFFWARNYes (+warning returned)

tidb_nontransactional_ignore_error New in v6.1.0

  • Scope: SESSION | GLOBAL
  • Persists to cluster: Yes
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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_desc_factor

  • Scope: SESSION | GLOBAL
  • Persists to cluster: 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
  • 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
  • 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
  • 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_force_inline_cte New in v6.3.0

  • Scope: SESSION | GLOBAL
  • Persists to cluster: 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_insubq_to_join_and_agg

  • Scope: SESSION | GLOBAL

  • Persists to cluster: 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
  • 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
  • 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
  • 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
  • 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
  • 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_prefer_range_scan New in v5.0

  • Scope: SESSION | GLOBAL
  • Persists to cluster: 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
  • 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
  • Type: Boolean
  • Default value: OFF
  • Specifies whether to allow the optimizer to push Projection down to the TiKV or TiFlash coprocessor.

tidb_opt_range_max_size New in v6.4.0

  • Scope: SESSION | GLOBAL
  • Persists to cluster: 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
  • 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
  • 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

  • Scope: SESSION | GLOBAL
  • Persists to cluster: 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
  • 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
  • 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_write_row_id

  • Scope: SESSION
  • 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
  • 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
  • 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

tidb_placement_mode New in v6.0.0

  • Scope: SESSION | GLOBAL
  • Persists to cluster: Yes
  • 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.
  • 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_pprof_sql_cpu New in v4.0

  • Scope: GLOBAL
  • Persists to cluster: No, only applicable to the current TiDB instance that you are connecting to.
  • 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_prepared_plan_cache_memory_guard_ratio New in v6.1.0

  • Scope: GLOBAL
  • Persists to cluster: Yes
  • 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

  • Scope: SESSION | GLOBAL
  • Persists to cluster: Yes
  • 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

  • Scope: SESSION | GLOBAL
  • Persists to cluster: Yes
  • 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
  • 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

  • Scope: GLOBAL
  • Persists to cluster: No, only applicable to the current TiDB instance that you are connecting to.
  • 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

  • Scope: SESSION | GLOBAL
  • Persists to cluster: Yes
  • 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
  • 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
  • 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

  • Scope: GLOBAL
  • Persists to cluster: No, only applicable to the current TiDB instance that you are connecting to.
  • 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

  • Scope: SESSION | GLOBAL
  • Persists to cluster: Yes
  • Type: Boolean
  • Default value: OFF
  • This variable controls whether to hide user information in the SQL statement being recorded into the TiDB log and slow log.
  • When you set the variable to 1, 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.

tidb_regard_null_as_point New in v5.4.0

  • Scope: SESSION | GLOBAL
  • Persists to cluster: Yes
  • 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
  • Type: Boolean
  • Default value: OFF
  • Specifies whether to remove ORDER BY clause in a subquery.

tidb_replica_read New in v4.0

  • Scope: SESSION | GLOBAL
  • Persists to cluster: Yes
  • Type: Enumeration
  • Default value: leader
  • Possible values: leader, follower, leader-and-follower, closest-replicas, closest-adaptive
  • 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

  • Scope: GLOBAL
  • Persists to cluster: Yes
  • 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:
  • 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.
  • When this variable is enabled, the SQL statements being executed are not affected. TiDB only performs the read-only check for the SQL statements to be executed.
  • 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_retry_limit

  • Scope: SESSION | GLOBAL
  • Persists to cluster: Yes
  • 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

  • Scope: SESSION | GLOBAL
  • Persists to cluster: Yes
  • Type: Integer
  • Default value: 1
  • 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 4.0.0 to 4.0.0, 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_scatter_region

  • Scope: GLOBAL
  • Persists to cluster: Yes
  • 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_server_memory_limit New in v6.4.0

  • Scope: GLOBAL
  • Persists to cluster: Yes
  • 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 in bytes. The value range is [0, 9223372036854775807]. The memory format with the units "KB|MB|GB|TB" is supported. The 0 value means no memory limit.
  • 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

  • Scope: GLOBAL
  • Persists to cluster: Yes
  • 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

  • Scope: GLOBAL
  • Persists to cluster: Yes
  • Default value: 134217728 (which is 128 MB)
  • Range: [128, 9223372036854775807], in bytes. The memory format with the units "KB|MB|GB|TB" 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_shard_allocate_step New in v5.0

  • Scope: SESSION | GLOBAL
  • Persists to cluster: Yes
  • 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

  • Scope: GLOBAL
  • Persists to cluster: Yes
  • 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
  • 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
  • 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_utf8_check

  • Scope: SESSION | GLOBAL
  • Persists to cluster: Yes
  • 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.

tidb_slow_log_threshold

  • Scope: GLOBAL
  • Persists to cluster: No, only applicable to the current TiDB instance that you are connecting to.
  • Type: Integer
  • Default value: 300
  • Range: [-1, 9223372036854775807]
  • Unit: Milliseconds
  • This variable is used to output the threshold value of the time consumed by the slow log. When the time consumed by a query is larger than this value, this query is considered as a slow log and its log is output to the slow query log.

tidb_slow_query_file

  • Scope: SESSION
  • 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
  • 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
  • Type: Integer
  • Default value: 1
  • Range: [1, 15]

tidb_stats_cache_mem_quota New in v6.1.0

  • Scope: GLOBAL
  • Persists to cluster: Yes
  • Type: Integer
  • Default value: 0
  • 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
  • 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

  • Scope: SESSION | GLOBAL
  • Persists to cluster: 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_history_size New in v4.0

  • Scope: GLOBAL
  • Persists to cluster: Yes
  • 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

  • Scope: GLOBAL
  • Persists to cluster: Yes
  • 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

  • Scope: GLOBAL
  • Persists to cluster: Yes
  • Type: Integer
  • Default value: 4096
  • Range: [0, 2147483647]
  • This variable is used to control the length of the SQL string in statement summary tables.

tidb_stmt_summary_max_stmt_count New in v4.0

  • Scope: GLOBAL
  • Persists to cluster: Yes
  • 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

  • Scope: GLOBAL
  • Persists to cluster: Yes
  • 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
  • Type: Integer
  • Default value: 0
  • 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
  • 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
  • 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
  • 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
  • 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

  • Scope: GLOBAL
  • Persists to cluster: Yes
  • Type: Integer
  • Default value: 1
  • Range: [1, 256]
  • 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
  • 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
  • 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

  • Scope: GLOBAL
  • Persists to cluster: Yes
  • 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.
  • 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

  • Scope: GLOBAL
  • Persists to cluster: Yes
  • 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.
  • 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
  • Type: Boolean
  • Default value: ON
  • This variable controls whether TiDB tracks the memory usage of aggregate functions.

tidb_tso_client_batch_max_wait_time New in v5.3.0

  • Scope: GLOBAL
  • Persists to cluster: Yes
  • Type: Float
  • Default value: 0
  • Range: [0, 10]
  • 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.

tidb_ttl_delete_rate_limit New in v6.5.0

  • Scope: GLOBAL
  • Persists to cluster: Yes
  • 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.

tidb_ttl_delete_batch_size New in v6.5.0

  • Scope: GLOBAL
  • Persists to cluster: Yes
  • 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.

tidb_ttl_delete_worker_count New in v6.5.0

  • Scope: GLOBAL
  • Persists to cluster: Yes
  • Default value: 4
  • Range: [1, 256]
  • This variable is used to set the maximum concurrency of TTL jobs on each TiDB node.

tidb_ttl_job_enable New in v6.5.0

  • Scope: GLOBAL
  • Persists to cluster: Yes
  • 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.

tidb_ttl_scan_batch_size New in v6.5.0

  • Scope: GLOBAL
  • Persists to cluster: Yes
  • 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.

tidb_ttl_scan_worker_count New in v6.5.0

  • Scope: GLOBAL
  • Persists to cluster: Yes
  • Default value: 4
  • Range: [1, 256]
  • This variable is used to set the maximum concurrency of TTL scan jobs on each TiDB node.

tidb_ttl_job_run_interval New in v6.5.0

  • Scope: GLOBAL
  • Persists to cluster: Yes
  • Default value: 1h0m0s
  • Range: [10m0s, 8760h0m0s]
  • This variable is used to control the scheduling interval of TTL jobs in the background. For example, if the current value is set to 1h0m0s, each table with TTL attributes cleans up expired data once every hour.

tidb_ttl_job_schedule_window_start_time New in v6.5.0

  • Scope: GLOBAL
  • 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.

tidb_ttl_job_schedule_window_end_time New in v6.5.0

  • Scope: GLOBAL
  • 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.

tidb_txn_assertion_level New in v6.0.0

  • Scope: SESSION | GLOBAL

  • Persists to cluster: Yes

  • 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
  • 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.
  • 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 maximum size of a single log (which is 8 MB by default), the commits might fail.

tidb_txn_mode

  • Scope: SESSION | GLOBAL
  • Persists to cluster: Yes
  • 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
  • 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

  • Scope: SESSION
  • 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

  • Scope: SESSION
  • 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

  • Scope: SESSION | GLOBAL
  • Persists to cluster: Yes
  • 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
  • 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
  • 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

  • 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 set to 8. 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.

time_zone

  • Scope: SESSION | GLOBAL
  • Persists to cluster: Yes
  • 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
  • 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
  • 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

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
  • 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

  • Scope: SESSION
  • 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
  • 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
  • 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

  • Scope: GLOBAL
  • Persists to cluster: Yes
  • 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
  • Type: Integer
  • Default value: 8
  • Range: [0, 2147483647]
  • 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
  • Type: Integer
  • Default value: 1
  • Range: [0, 2147483647]
  • 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
  • Type: Integer
  • Default value: 1
  • Range: [0, 2147483647]
  • 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
  • Type: Enumeration
  • Default value: 1
  • Value options: 0, 1, 2
  • 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
  • Type: Integer
  • Default value: 1
  • Range: [0, 2147483647]
  • 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
  • Default value: 5.7.25-TiDB-(tidb version)
  • This variable returns the MySQL version, followed by the TiDB version. For example '5.7.25-TiDB-v4.0.0-beta.2-716-g25e003253'.

version_comment

  • Scope: NONE
  • Default value: (string)
  • This variable returns additional details about the TiDB version. For example, 'TiDB Server (Apache License 2.0) Community Edition, MySQL 5.7 compatible'.

version_compile_machine

  • Scope: NONE
  • Default value: (string)
  • This variable returns the name of the CPU architecture on which TiDB is running.

version_compile_os

  • Scope: NONE
  • Default value: (string)
  • This variable returns the name of the OS on which TiDB is running.

wait_timeout

  • Scope: SESSION | GLOBAL
  • Persists to cluster: Yes
  • 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
  • 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
  • Type: Boolean
  • Default value: ON
  • This variable controls whether to use the high precision mode when computing the window functions.
Download PDFRequest docs changesAsk questions on TiDB Forum
Was this page helpful?
Open Source Ecosystem
TiDB
TiKV
TiSpark
Chaos Mesh
© 2023 PingCAP. All Rights Reserved.