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TiDB Specific System Variables

TiDB contains a number of system variables which are specific to its usage, and do not apply to MySQL. These variables start with a tidb_ prefix, and can be tuned to optimize system performance.

System variables

Variables can be set with the SET statement, for example:

set @@tidb_distsql_scan_concurrency = 10

If you need to set the global variable, run:

set @@global.tidb_distsql_scan_concurrency = 10

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_import_data

  • Scope: SESSION
  • Default value: 0
  • This variable indicates whether to import data from the dump file currently.
  • To speed up importing, the unique index constraint is not checked when the variable is set to 1.
  • This variable is only used by Lightning. Do not modify it.

tidb_opt_agg_push_down

  • Scope: SESSION
  • Default value: 0
  • This variable is used to set whether the optimizer executes the optimization operation of pushing down the aggregate function to the position before Join.
  • When the aggregate operation is slow in query, you can set the variable value to 1.

tidb_opt_insubquery_unfold

  • Scope: SESSION | GLOBAL
  • Default value: 0
  • This variable is used to set whether the optimizer executes the optimization operation of unfolding the "in-" subquery.

tidb_build_stats_concurrency

  • Scope: SESSION
  • Default value: 4
  • 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_checksum_table_concurrency

  • Scope: SESSION
  • Default value: 4
  • 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_current_ts

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

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_distsql_scan_concurrency

  • Scope: SESSION | GLOBAL
  • Default value: 15
  • 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 cannot exceed the number of CPU cores of all the TiKV nodes.

tidb_index_lookup_size

  • Scope: SESSION | GLOBAL
  • Default value: 20000
  • 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_lookup_concurrency

  • Scope: SESSION | GLOBAL
  • Default value: 4
  • 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.

tidb_index_lookup_join_concurrency

  • Scope: SESSION | GLOBAL
  • Default value: 4
  • This variable is used to set the concurrency of the index lookup join algorithm.

tidb_hash_join_concurrency

  • Scope: SESSION | GLOBAL
  • Default value: 5
  • This variable is used to set the concurrency of the hash join algorithm.

tidb_index_serial_scan_concurrency

  • Scope: SESSION | GLOBAL
  • Default value: 1
  • 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_projection_concurrency

  • Scope: SESSION | GLOBAL
  • Default value: 4
  • This variable is used to set the concurrency of the Projection operator.

tidb_hashagg_partial_concurrency

  • Scope: SESSION | GLOBAL
  • Default value: 4
  • 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.

tidb_hashagg_final_concurrency

  • Scope: SESSION | GLOBAL
  • Default value: 4
  • 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.

tidb_index_join_batch_size

  • Scope: SESSION | GLOBAL
  • Default value: 25000
  • 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_skip_utf8_check

  • Scope: SESSION | GLOBAL
  • Default value: 0
  • 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 1.

tidb_max_chunk_size

  • Scope: SESSION | GLOBAL
  • Default value: 1024
  • This variable is used to set the maximum number of rows in a chunk during the execution process.

tidb_mem_quota_query

  • Scope: SESSION
  • Default value: 32 GB
  • 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.

tidb_mem_quota_hashjoin

  • Scope: SESSION
  • Default value: 32 GB
  • This variable is used to set the threshold value of memory quota for the HashJoin operator.
  • If the memory quota of the HashJoin operator during execution exceeds the threshold value, TiDB performs the operation designated by the OOMAction option in the configuration file.

tidb_mem_quota_mergejoin

  • Scope: SESSION
  • Default value: 32 GB
  • This variable is used to set the threshold value of memory quota for the MergeJoin operator.
  • If the memory quota of the MergeJoin operator during execution exceeds the threshold value, TiDB performs the operation designated by the OOMAction option in the configuration file.

tidb_mem_quota_sort

  • Scope: SESSION
  • Default value: 32 GB
  • This variable is used to set the threshold value of memory quota for the Sort operator.
  • If the memory quota of the Sort operator during execution exceeds the threshold value, TiDB performs the operation designated by the OOMAction option in the configuration file.

tidb_mem_quota_topn

  • Scope: SESSION
  • Default value: 32 GB
  • This variable is used to set the threshold value of memory quota for the TopN operator.
  • If the memory quota of the TopN operator during execution exceeds the threshold value, TiDB performs the operation designated by the OOMAction option in the configuration file.

tidb_mem_quota_indexlookupreader

  • Scope: SESSION
  • Default value: 32 GB
  • This variable is used to set the threshold value of memory quota for the IndexLookupReader operator.
  • If the memory quota of the IndexLookupReader operator during execution exceeds the threshold value, TiDB performs the operation designated by the OOMAction option in the configuration file.

tidb_mem_quota_indexlookupjoin

  • Scope: SESSION
  • Default value: 32 GB
  • This variable is used to set the threshold value of memory quota for the IndexLookupJoin operator.
  • If the memory quota of the IndexLookupJoin operator during execution exceeds the threshold value, TiDB performs the operation designated by the OOMAction option in the configuration file.

tidb_mem_quota_nestedloopapply

  • Scope: SESSION
  • Default value: 32 GB
  • This variable is used to set the threshold value of memory quota for the NestedLoopApply operator.
  • If the memory quota of the NestedLoopApply operator during execution exceeds the threshold value, TiDB performs the operation designated by the OOMAction option in the configuration file.

tidb_general_log

  • Scope: SERVER
  • Default value: 0
  • This variable is used to set whether to record all the SQL statements in the log.

tidb_enable_streaming

  • Scope: SERVER
  • Default value: 0
  • This variable is used to set whether to enable Streaming.

tidb_retry_limit

  • Scope: SESSION | GLOBAL
  • Default value: 10
  • This variable is used to set the maximum number of the retries. 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.

tidb_disable_txn_auto_retry

  • Scope: SESSION | GLOBAL

  • Default: off

  • This variable is used to set whether to disable the automatic retry of explicit 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. The automatic retry is enabled by default in TiDB v2.1.

    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.

tidb_backoff_weight

  • Scope: SESSION | GLOBAL

  • Default value: 2

  • 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_enable_table_partition

  • Scope: SESSION
  • Default value: 0
  • This variable is used to set whether to enable the TABLE PARTITION feature.

tidb_backoff_lock_fast

  • Scope: SESSION | GLOBAL
  • Default value: 100
  • This variable is used to set the backoff time when the read request meets a lock.

tidb_ddl_reorg_worker_cnt

  • Scope: GLOBAL
  • Default value (before 2.1.17): 16
  • Default value (in 2.1.17 or later): 4
  • This variable is used to set the concurrency of the DDL operation in the re-organize phase.

tidb_ddl_reorg_batch_size

  • Scope: GLOBAL
  • Default value (before 2.1.17): 1024
  • Default value (in 2.1.17 or later): 256
  • 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. The maximum value is 10240. 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_max_delta_schema_count New in v2.1.18

  • Scope: GLOBAL
  • Default value: 1024
  • 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. This variable is supported in TiDB 2.1.18 and later versions.

tidb_force_priority

  • Scope: SESSION
  • 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.

SHARD_ROW_ID_BITS

For the tables with non-integer PK or without PK, TiDB uses an implicit auto-increment ROW ID. When a large number of INSERT operations occur, the data is written into a single Region, causing a write hot spot.

To mitigate the hot spot issue, you can configure SHARD_ROW_ID_BITS. The ROW ID is scattered and the data is written into multiple different Regions. But setting an overlarge value might lead to an excessively large number of RPC requests, which increases the CPU and network overheads.

  • SHARD_ROW_ID_BITS = 4 indicates 16 shards
  • SHARD_ROW_ID_BITS = 6 indicates 64 shards
  • SHARD_ROW_ID_BITS = 0 indicates the default 1 shard

Usage of statements:

  • CREATE TABLE: CREATE TABLE t (c int) SHARD_ROW_ID_BITS = 4;
  • ALTER TABLE: ALTER TABLE t SHARD_ROW_ID_BITS = 4;

tidb_slow_log_threshold

  • Scope: SESSION
  • Default value: 300ms
  • 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.

Usage example:

set tidb_slow_log_threshold = 200

tidb_query_log_max_len

  • Scope: SESSION
  • Default value: 2048 (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_scatter_region

  • Scope: GLOBAL
  • Default value: 0
  • 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.

tidb_allow_remove_auto_inc New in v2.1.18

  • Scope: SESSION
  • Default value: 0
  • 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.
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