Important

You are viewing the documentation of an older version of the TiDB database (TiDB v5.4). It is recommended that you use the latest LTS version of the TiDB database.

System Variables

TiDB system variables behave similar to MySQL with some differences, in that settings might apply on a SESSION, INSTANCE, or GLOBAL scope, or on a scope that combines SESSION, INSTANCE, or GLOBAL.

Changes to GLOBAL scoped variables only apply to new connection sessions with TiDB. Currently active connection sessions are not affected. These changes are persisted and valid after restarts.
Changes to INSTANCE scoped variables apply to all active or new connection sessions with the current TiDB instance immediately after the changes are made. Other TiDB instances are not affected. These changes are not persisted and become invalid after TiDB restarts.
Variables can also have NONE scope. These variables are read-only, and are typically used to convey static information that will not change after a TiDB server has started.

Variables can be set with the SET statement on a per-session, instance or global basis:

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

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

Note

Executing SET GLOBAL applies immediately on the TiDB server where the statement was issued. A notification is then sent to all TiDB servers to refresh their system variable cache, which will start immediately as a background operation. Because there is a risk that some TiDB servers might miss the notification, the system variable cache is also refreshed automatically every 30 seconds. This helps ensure that all servers are operating with the same configuration.

TiDB differs from MySQL in that GLOBAL scoped variables persist through TiDB server restarts. Additionally, TiDB presents several MySQL variables as both readable and settable. This is required for compatibility, because it is common for both applications and connectors to read MySQL variables. For example, JDBC connectors both read and set query cache settings, despite not relying on the behavior.

Note

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

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

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

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

Variable Reference

allow_auto_random_explicit_insert New in v4.0.3

Scope: SESSION | GLOBAL
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
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
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
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
Default value: aes-128-ecb
Defines the encryption mode for the AES_ENCRYPT() and AES_DECRYPT() functions.

character_set_client

Scope: SESSION | GLOBAL
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
Default value: utf8mb4
The character set for string literals that do not have a specified character set.

character_set_database

Scope: SESSION | GLOBAL
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
Default value: utf8mb4
The character set that is used when data is sent to the client.

character_set_server

Scope: SESSION | GLOBAL
Default value: utf8mb4
The default character set for the server.

collation_connection

Scope: SESSION | GLOBAL
Default value: utf8mb4_bin
This variable indicates the collation for string literals that do not have a specified collation.

collation_database

Scope: SESSION | GLOBAL
Default value: utf8mb4_bin
This variable indicates the collation 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.

collation_server

Scope: SESSION | GLOBAL
Default value: utf8mb4_bin
The default collation for the server.

cte_max_recursion_depth

Scope: SESSION | GLOBAL
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: INSTANCE
Default value: 300
Unit: Milliseconds
Log DDL operations whose execution time exceeds the threshold value.

default_authentication_plugin

Scope: GLOBAL
Default value: mysql_native_password
Possible values: mysql_native_password, caching_sha2_password
This variable sets the authentication method that the server advertises when the server-client connection is being established. Possible values for this variable are documented in Authentication plugin status.
Value options: mysql_native_password and caching_sha2_password. For more details, see Authentication plugin status.

default_week_format

Scope: SESSION | GLOBAL
Default value: 0
Range: [0, 7]
Sets the week format used by the WEEK() function.

error_count

Scope: NONE
Type: Integer
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
Default value: OFF
For compatibility, TiDB returns foreign key checks as OFF.

group_concat_max_len

Scope: SESSION | GLOBAL
Default value: 1024
Range: [4, 18446744073709551615]
The maximum buffer size for items in the GROUP_CONCAT() function.

have_openssl

Scope: NONE
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
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

This variable is an alias for last_insert_id.

init_connect

Scope: GLOBAL
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
Default value: 50
Range: [1, 1073741824]
Unit: Seconds
The lock wait timeout for pessimistic transactions (default).

interactive_timeout

Scope: SESSION | GLOBAL
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
Default value: 0
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
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
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.

license

Scope: NONE
Default value: Apache License 2.0
This variable indicates the license of your TiDB server installation.

log_bin

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

max_allowed_packet

Scope: SESSION | GLOBAL
Default value: 67108864
Range: [1024, 1073741824]
Unit: Bytes
The maximum size of a packet for the MySQL protocol.

max_connections

Scope: GLOBAL
Persists to cluster: No
Type: Integer
Default value: 0
Range: [0, 100000]
The maximum number of connections permitted for a single TiDB instance.
The value of 0 means no limit.

max_execution_time

Scope: SESSION | GLOBAL
Default value: 0
Range: [0, 2147483647]
Unit: Milliseconds
The maximum execution time of a statement. The default value is unlimited (zero).

Note

Unlike in MySQL, the max_execution_time system variable currently works on all kinds of statements in TiDB, not only restricted to the SELECT statement. The precision of the timeout value is roughly 100ms. This means the statement might not be terminated in accurate milliseconds as you specify.

placement_checks New in v5.3.0

Scope: SESSION | GLOBAL
Default value: ON

This variable controls whether DDL statements validate Placement Rules in SQL.

It is intended to be used by logical dump/restore tools to ensure that tables can always be created even if placement rules are violated. This is similar to how mysqldump writes SET FOREIGN_KEY_CHECKS=0; to the start of every dump file.

plugin_dir

Scope: INSTANCE
Default value: ""
Indicates the directory to load plugins as specified by a command-line flag.

plugin_load

Scope: INSTANCE
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
Default value: 4000
Range: [0, 65535]
The port that the tidb-server is listening on when speaking the MySQL protocol.

rand_seed1

Scope: SESSION
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
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.

skip_name_resolve New in v5.2.0

Scope: GLOBAL
Default value: OFF
This variable controls whether the tidb-server instance resolves hostnames as a part of the connection handshake.
When the DNS is unreliable, you can enable this option to improve network performance.

Note

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

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

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

socket

Scope: NONE
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
Default value: ON
Indicates whether to write changes to TiDB Binlog or not.

Note

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

sql_mode

Scope: SESSION | GLOBAL
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_select_limit New in v4.0.2

Scope: SESSION | GLOBAL
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.

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.

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_allow_batch_cop New in v4.0

Scope: SESSION | GLOBAL
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
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: lower, md5, reverse, 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
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
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_version New in v5.1.0

Scope: SESSION | GLOBAL
Default value: 2
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
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_ratio

Scope: GLOBAL
Default value: 0.5
This variable is used to set the threshold when TiDB automatically executes ANALYZE TABLE in a background thread to update table statistics. For example, a value of 0.5 means that auto-analyze is triggered when greater than 50% of the rows in a table have been modified. Auto-analyze can be restricted to only execute during certain hours of the day by specifying tidb_auto_analyze_start_time and tidb_auto_analyze_end_time.

Note

Only when the run-auto-analyze option is enabled in the starting configuration file of TiDB, the auto_analyze feature can be triggered.

tidb_auto_analyze_start_time

Scope: GLOBAL
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 1AM and 3AM, set tidb_auto_analyze_start_time='01:00 +0000' and tidb_auto_analyze_end_time='03:00 +0000'.

tidb_backoff_lock_fast

Scope: SESSION | GLOBAL
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
Default value: 2
Range: [1, 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_insert

Scope: SESSION
Default value: OFF
This variable permits tidb_dml_batch_size to be used in INSERT statements.
Only the value OFF provides ACID compliance. Setting this to any other value breaks the atomicity and isolation guarantees of TiDB, because an individual INSERT statement will be split into smaller transactions.

tidb_broadcast_join_threshold_count New in v5.0

Scope: SESSION | GLOBAL
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
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
Default value: 4
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: SESSION | GLOBAL
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_check_mb4_value_in_utf8

Scope: INSTANCE
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
Default value: 4
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_config

Scope: SESSION
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
Default value: OFF

This setting only applies to optimistic transactions. 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 set to zero 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 'PRIMARY'

When set to 1 and using optimistic transactions:

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

Constraint checking is always performed in place for pessimistic transactions (default).

tidb_current_ts

Scope: SESSION
Default value: 0
This variable is read-only. It is used to obtain the timestamp of the current transaction.

tidb_ddl_error_count_limit

Scope: GLOBAL
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_reorg_batch_size

Scope: GLOBAL
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
Default value: PRIORITY_LOW
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
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_disable_txn_auto_retry

Scope: SESSION | GLOBAL
Default value: ON
This variable is used to set whether to disable the automatic retry of explicit optimistic transactions. The default value of ON means that transactions will not automatically retry in TiDB and COMMIT statements might return errors that need to be handled in the application layer.
Setting the value to OFF means that TiDB will automatically retry transactions, resulting in fewer errors from COMMIT statements. Be careful when making this change, because it might result in lost updates.
This variable does not affect automatically committed implicit transactions and internally executed transactions in TiDB. The maximum retry count of these transactions is determined by the value of tidb_retry_limit.
For more details, see limits of retry.
This variable only applies to optimistic transactions, not to pessimistic transactions. The number of retries for pessimistic transactions is controlled by max_retry_count.

tidb_distsql_scan_concurrency

Scope: SESSION | GLOBAL
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
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.
To use tidb_dml_batch_size in INSERT statements, you also need to set the system variable tidb_batch_insert to ON.
Only the value 0 provides ACID compliance. Setting this to any other value will break the atomicity and isolation guarantees of TiDB.

tidb_enable_1pc New in v5.0

Scope: SESSION | GLOBAL
Default value: ON
This variable is used to specify whether to enable the one-phase commit feature for transactions that only affect one Region. Compared with the often-used two-phase commit, one-phase commit can greatly reduce the latency of transaction commit and increase the throughput.

Note

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

tidb_enable_alter_placement

Warning

Currently, Placement Rules in SQL is an experimental feature. It is not recommended that you use it in production environments.

Scope: GLOBAL
Default value: OFF

This variable enables or disables Placement Rules in SQL.

tidb_enable_amend_pessimistic_txn New in v4.0.7

Scope: SESSION | GLOBAL
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.

Note

Currently, this feature is incompatible with TiDB Binlog in some scenarios and might cause semantic changes on a transaction. For more usage precautions of this feature, refer to Incompatibility issues about transaction semantic and Incompatibility issues about TiDB Binlog.

tidb_enable_async_commit New in v5.0

Scope: SESSION | GLOBAL
Default value: ON
This variable controls whether to enable the async commit feature for the second phase of the two-phase transaction commit to perform asynchronously in the background. Enabling this feature can reduce the latency of transaction commit.

Note

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

tidb_enable_auto_increment_in_generated

Scope: SESSION | GLOBAL
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_change_multi_schema

Warning

TiDB will support more types of multi-schema changes in the future. This system variable will be deleted in a future release of TiDB.

Scope: GLOBAL
Persists to cluster: Yes
Type: Boolean
Default value: OFF
This variable is used to control whether multiple columns or indexes can be altered in one ALTER TABLE statement. When the value of this variable is ON, only the following multi-schema changes are supported:
- Add multiple columns. For example, ALTER TABLE t ADD COLUMN c1 INT, ADD COLUMN c2 INT;.
- Drop multiple columns. For example, ALTER TABLE t DROP COLUMN c1, DROP COLUMN c2;.
- Drop multiple indexes. For example, ALTER TABLE t DROP INDEX i1, DROP INDEX i2;.
- Drop a column covered by a single-column index. For example, ALTER TABLE t DROP COLUMN c1, in which the schema contains INDEX idx(c1).

tidb_enable_cascades_planner

Warning

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

Scope: SESSION | GLOBAL
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
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
Default value: INT_ONLY
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_collect_execution_info

Scope: INSTANCE
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

Warning

Currently, collecting statistics on PREDICATE COLUMNS is an experimental feature. It is not recommended that you use it in production environments.

Scope: GLOBAL
Default value: OFF
This variable controls whether to enable TiDB to collect PREDICATE COLUMNS. After enabling the collection, if you disable it, the information of previously collected PREDICATE COLUMNS is cleared. For details, see Collect statistics on some columns.

tidb_enable_enhanced_security

Scope: NONE
Default value: OFF
This variable indicates whether the TiDB server you are connected to has the Security Enhanced Mode (SEM) enabled. To change its value, you need to modify the value of enable-sem in your TiDB server configuration file and restart the TiDB server.
SEM is inspired by the design of systems such as Security-Enhanced Linux. It reduces the abilities of users with the MySQL SUPER privilege and instead requires RESTRICTED fine-grained privileges to be granted as a replacement. These fine-grained privileges include:
- RESTRICTED_TABLES_ADMIN: The ability to write data to system tables in the mysql schema and to see sensitive columns on information_schema tables.
- RESTRICTED_STATUS_ADMIN: The ability to see sensitive variables in the command SHOW STATUS.
- RESTRICTED_VARIABLES_ADMIN: The ability to see and set sensitive variables in SHOW [GLOBAL] VARIABLES and SET.
- RESTRICTED_USER_ADMIN: The ability to prevent other users from making changes or dropping a user account.

tidb_enable_fast_analyze

Warning

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

Scope: SESSION | GLOBAL
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_index_merge New in v4.0

Scope: SESSION | GLOBAL
Default value: ON
This variable is used to control whether to enable the index merge feature.

Note

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

tidb_enable_list_partition New in v5.0

Warning

Currently, List partition and List COLUMNS partition are experimental features. It is not recommended that you use it in production environments.

Scope: SESSION | GLOBAL
Default value: OFF
This variable is used to set whether to enable the LIST (COLUMNS) TABLE PARTITION feature.

tidb_enable_noop_functions New in v4.0

Scope: SESSION | GLOBAL
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:
- get_lock and release_lock functions
- LOCK IN SHARE MODE syntax
- SQL_CALC_FOUND_ROWS syntax
- START TRANSACTION READ ONLY and SET TRANSACTION READ ONLY syntax
- The tx_read_only, transaction_read_only, offline_mode, super_read_only, read_only and sql_auto_is_null system variables
- GROUP BY <expr> ASC|DESC syntax

Warning

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

tidb_enable_paging New in v5.4.0

Scope: SESSION | GLOBAL
Default value: OFF
This variable controls whether to use the method of paging to send coprocessor requests in IndexLookUp operator.
User scenarios: 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 CPU usage for TiKV's unified read pool. 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.
When tidb_enable_paging is enabled, for the IndexLookUp requests with Limit that cannot be pushed down and are fewer than 960, TiDB uses the method of paging to send coprocessor requests. The fewer Limit, the more obvious the optimization.

tidb_enable_parallel_apply New in v5.0

Scope: SESSION | GLOBAL
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_pseudo_for_outdated_stats New in v5.3.0

Scope: SESSION | GLOBAL
Default value: ON
This variable controls the behavior of the optimizer on using statistics of a table when the statistics are outdated.

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

By default (with the variable value ON), when the statistics of a table is outdated, 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 you set the variable value to OFF, even if the statistics of a table are outdated, the optimizer still keeps using the statistics.
If the data on a table is frequently modified without executing ANALYZE on this table in time, to keep the execution plan stable, you can set the variable value to OFF.

tidb_enable_rate_limit_action

Note

This variable is enabled by default, which makes the memory usage not under the control of tidb_mem_quota_query in some cases. Therefore, it is recommended to set the value of tidb_enable_rate_limit_action to OFF.

Scope: SESSION | GLOBAL
Default value: ON
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 continues to exceed tidb_mem_quota_query, this SQL statement triggers other memory control behaviors, such as spilling data to disk.

tidb_enable_slow_log

Scope: INSTANCE
Default value: ON
This variable is used to control whether to enable the slow log feature.

tidb_enable_stmt_summary New in v3.0.4

Scope: SESSION | GLOBAL
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
Default value: ON
This variable is used to control if tables can be created with invalid definitions of type DOUBLE. This setting is intended to provide an upgrade path from earlier versions of TiDB, which were less strict in validating types.
The default value of ON is compatible with MySQL.

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

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

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

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

Note

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

tidb_enable_table_partition

Scope: SESSION | GLOBAL
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
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.

tidb_enable_tso_follower_proxy New in v5.3.0

Scope: GLOBAL
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.

Note

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

tidb_enable_vectorized_expression New in v4.0

Scope: SESSION | GLOBAL
Default value: ON
This variable is used to control whether to enable vectorized execution.

tidb_enable_window_function

Scope: SESSION | GLOBAL
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
Default value: OFF

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

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

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
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
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
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
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
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: INSTANCE
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: INSTANCE
Default value: NO_PRIORITY
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.
You can set the value of this variable to NO_PRIORITY, LOW_PRIORITY, DELAYED or HIGH_PRIORITY.

tidb_gc_concurrency New in v5.0

Scope: GLOBAL
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
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
Default value: 10m0s
Range: [10m0s, 8760h0m0s]
The time limit during which data is retained for each GC, in the format of Go Duration. When a GC happens, the current time minus this value is the safe point.

Note

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

tidb_gc_run_interval New in v5.0

Scope: GLOBAL
Default value: 10m0s
Range: [10m0s, 8760h0m0s]
Specifies the GC interval, in the format of Go Duration, for example, "1h30m", and "15m"

tidb_gc_scan_lock_mode New in v5.0

Warning

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

Scope: GLOBAL
Default value: LEGACY
Possible values: PHYSICAL, LEGACY
- LEGACY: Uses the old way of scanning, that is, disable Green GC.
- PHYSICAL: Uses the physical scanning method, that is, enable Green GC.

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

tidb_general_log

Scope: INSTANCE
Default value: OFF

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

tidb_hash_join_concurrency

Warning

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

Scope: SESSION | GLOBAL
Default value: -1
Range: [1, 256]
Unit: Threads
This variable is used to set the concurrency of the hash join algorithm.
A value of -1 means that the value of tidb_executor_concurrency will be used instead.

tidb_hashagg_final_concurrency

Warning

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

Scope: SESSION | GLOBAL
Default value: -1
Range: [1, 256]
Unit: Threads
This variable is used to set the concurrency of executing the concurrent hash aggregation algorithm in the final phase.
When the parameter of the aggregate function is not distinct, HashAgg is run concurrently and respectively in two phases - the partial phase and the final phase.
A value of -1 means that the value of tidb_executor_concurrency will be used instead.

tidb_hashagg_partial_concurrency

Warning

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

Scope: SESSION | GLOBAL
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_index_join_batch_size

Scope: SESSION | GLOBAL
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

Warning

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

Scope: SESSION | GLOBAL
Default value: -1
Range: [1, 256]
Unit: Threads
This variable is used to set the concurrency of the index lookup operation.
Use a bigger value in OLAP scenarios, and a smaller value in OLTP scenarios.
A value of -1 means that the value of tidb_executor_concurrency will be used instead.

tidb_index_lookup_join_concurrency

Warning

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

Scope: SESSION | GLOBAL
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_lookup_size

Scope: SESSION | GLOBAL
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
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
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_log_file_max_days New in v5.3.0

Scope: SESSION
Default value: 0
Range: [0, 2147483647]

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

tidb_low_resolution_tso

Scope: SESSION
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_chunk_size

Scope: SESSION | GLOBAL
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
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_mem_quota_apply_cache New in v5.0

Scope: SESSION | GLOBAL
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_query

Scope: SESSION
Default value: 1073741824 (1 GiB)
Range: [-1, 9223372036854775807]
Unit: Bytes
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 designated by the OOMAction option in the configuration file.

The initial value of this variable is configured by mem-quota-query.

tidb_memory_usage_alarm_ratio

Scope: INSTANCE
Default value: 0.8

TiDB triggers an alarm when the percentage of the memory it takes exceeds a certain threshold. For the detailed usage description of this feature, see memory-usage-alarm-ratio.
You can set the initial value of this variable by configuring memory-usage-alarm-ratio.

tidb_metric_query_range_duration New in v4.0

Scope: SESSION
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
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_multi_statement_mode New in v4.0.11

Scope: SESSION | GLOBAL
Default value: OFF
Possible values: OFF, ON, WARN
This variable controls whether to allow multiple queries to be executed in the same COM_QUERY call.
To reduce the impact of SQL injection attacks, TiDB now prevents multiple queries from being executed in the same COM_QUERY call by default. This variable is intended to be used as part of an upgrade path from earlier versions of TiDB. The following behaviors apply:

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

Note

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

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

tidb_opt_agg_push_down

Scope: SESSION
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_correlation_exp_factor

Scope: SESSION | GLOBAL
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
Default value: 0.9
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_distinct_agg_push_down

Scope: SESSION
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
Default value: ON
This variable is used to control whether the optimizer estimates the number of rows based on column order correlation

tidb_opt_insubq_to_join_and_agg

Scope: SESSION | GLOBAL
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_limit_push_down_threshold

Scope: SESSION | GLOBAL
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_prefer_range_scan New in v5.0

Scope: SESSION | GLOBAL
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_write_row_id

Scope: SESSION
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_partition_prune_mode New in v5.1

Warning

Currently, the dynamic pruning mode for partitioned tables is an experimental feature. It is not recommended that you use it in production environments.

Scope: SESSION | GLOBAL
Default value: static
Specifies whether to enable dynamic mode for partitioned tables. For details about the dynamic pruning mode, see Dynamic Pruning Mode for Partitioned Tables.

tidb_persist_analyze_options New in v5.4.0

Scope: GLOBAL
Default value: ON
This variable controls whether to enable the ANALYZE configuration persistence feature.

tidb_pprof_sql_cpu New in v4.0

Scope: INSTANCE
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_projection_concurrency

Warning

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

Scope: SESSION | GLOBAL
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: INSTANCE
Default value: 4096 (4 KiB)
Range: [-1, 9223372036854775807]
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.

Usage example:

SET tidb_query_log_max_len = 20

tidb_read_consistency New in v5.4.0

Scope: SESSION
Persists to cluster: No
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
Default value: 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: INSTANCE
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
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
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_replica_read New in v4.0

Scope: SESSION | GLOBAL
Default value: leader
Possible values: leader, follower, leader-and-follower, closest-replicas
This variable is used to control where TiDB reads data. Here are three options:
- leader: Read only from leader node
- follower: Read only from follower node
- leader-and-follower: Read from leader or follower node
See follower reads for additional details.

tidb_restricted_read_only New in v5.2.0

Warning

For TiDB versions earlier than v5.3.1 or v5.4.1, this variable has a defect and using it might cause unexpected results. Ensure that you use this variable ONLY in TiDB versions later than v5.3.1 or v5.4.1.

Scope: GLOBAL
Default value: OFF
tidb_restricted_read_only and tidb_super_read_only behave similarly. In most cases, you should use tidb_super_read_only only.
Users with the SUPER or SYSTEM_VARIABLES_ADMIN privilege can modify this variable. However, if the Security Enhanced Mode is enabled, the additional RESTRICTED_VARIABLES_ADMIN privilege is required to read or modify this variable.
tidb_restricted_read_only affects tidb_super_read_only in the following cases:
- Setting tidb_restricted_read_only to ON will update tidb_super_read_only to ON.
- Setting tidb_restricted_read_only to OFF leaves tidb_super_read_only unchanged.
- If tidb_restricted_read_only is ON, tidb_super_read_only cannot be set to OFF.
For DBaaS providers of TiDB, if a TiDB cluster is a downstream database of another database, to make the TiDB cluster read-only, you might need to use tidb_restricted_read_only with Security Enhanced Mode enabled, which prevents your customers from using tidb_super_read_only to make the cluster writable. To achieve this, you need to enable Security Enhanced Mode, use an admin user with the SYSTEM_VARIABLES_ADMIN and RESTRICTED_VARIABLES_ADMIN privileges to control tidb_restricted_read_only, and let your database users use the root user with the SUPER privilege to control tidb_super_read_only only.
This variable controls the read-only status of the entire cluster. When the variable is ON, all TiDB servers in the entire cluster are in the read-only mode. In this case, TiDB only executes the statements that do not modify data, such as SELECT, USE, and SHOW. For other statements such as INSERT and UPDATE, TiDB rejects executing those statements in the read-only mode.
Enabling the read-only mode using this variable only ensures that the entire cluster finally enters the read-only status. If you have changed the value of this variable in a TiDB cluster but the change has not yet propagated to other TiDB servers, the un-updated TiDB servers are still not in the read-only mode.
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
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: GLOBAL
Default value: 2
Range: [1, 2]
Controls the format version of the newly saved data in the table. In TiDB v4.0, the new storage row format version 2 is used by default to save new data.
If you upgrade from a TiDB version earlier than 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
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_skip_ascii_check New in v5.0

Scope: SESSION | GLOBAL
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
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
Default value: OFF
This variable is used to set whether to skip UTF-8 validation.
Validating UTF-8 characters affects the performance. When you are sure that the input characters are valid UTF-8 characters, you can set the variable value to ON.

Note

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

tidb_slow_log_threshold

Scope: INSTANCE
Default value: 300
Range: [-1, 9223372036854775807]
Unit: Milliseconds
This variable outputs the threshold value of the time consumed by the slow log, and is set to 300 milliseconds by default. When the time consumed by a query is larger than this value, this query is considered as a slow query and its log is output to the slow query log. Note that when the output level of log.level is "debug", all queries are recorded in the slow query log, regardless of the setting of this variable.

Usage example:

SET tidb_slow_log_threshold = 200;

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_stats_load_sync_wait New in v5.4.0

WARNING:
Currently, synchronously loading statistics is an experimental feature. It is not recommended that you use it in production environments.

Scope: SESSION | GLOBAL
Default value: 0
Unit: milliseconds
Range: [0, 4294967295]
This variable controls whether to enable the synchronously loading statistics feature. The default 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_stats_load_pseudo_timeout` New in v5.4.0

WARNING:
Currently, synchronously loading statistics is an experimental feature. It is not recommended that you use it in production environments.

Scope: GLOBAL
Default value: OFF
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 OFF means that SQL execution fails after the timeout. If you set this variable to ON, the SQL optimization gets back to using pseudo statistics after the timeout.

tidb_stmt_summary_history_size New in v4.0

Scope: SESSION | GLOBAL
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: SESSION | GLOBAL
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: SESSION | GLOBAL
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: SESSION | GLOBAL
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: SESSION | GLOBAL
Default value: 1800
Range: [1, 2147483647]
Unit: Seconds
This variable is used to set the refresh time of statement summary tables.

`tidb_enable_top_sql` New in v5.4.0

Scope: GLOBAL
Default value: OFF

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

Warning

Currently, Top SQL is an experimental feature. It is not recommended that you use it for production environments.

tidb_store_limit New in v3.0.4 and v4.0

Scope: GLOBAL
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_super_read_only New in v5.3.1

Warning

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.
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.
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_tmp_table_max_size New in v5.3.0

Scope: SESSION | GLOBAL
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_tso_client_batch_max_wait_time New in v5.3.0

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

Note

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

tidb_txn_mode

Scope: SESSION | GLOBAL
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
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
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
Default value: 300
Range: [1, 2147483647]
Unit: Seconds
This variable is used to set the timeout for executing the SPLIT REGION statement. If a statement is not executed completely within the specified time value, a timeout error is returned.

tidb_window_concurrency New in v4.0

Warning

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

Scope: SESSION | GLOBAL
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.

time_zone

Scope: SESSION | GLOBAL
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
Default value: 0
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
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.

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
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
Default value: ON
This variable controls whether to use the high precision mode when computing the window functions.

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