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TiDB 5.2 Release Notes

Release date: August 27, 2021

TiDB version: 5.2.0

In v5.2, the key new features and improvements are as follows:

  • Support using several functions in expression indexes to greatly improve query performance
  • Improve the accuracy of optimizer cardinality estimation to help to select optimal execution plans
  • Announce the general availability (GA) for the Lock View feature to observe transaction locking events and troubleshoot deadlock problems
  • Add the TiFlash I/O traffic limit feature to improve the stability of read and write for TiFlash
  • TiKV introduces a new flow control mechanism to replace the previous RocksDB write stall mechanism to improve the stability of TiKV flow control
  • Simplify the operation and maintenance of Data Migration (DM) to reduce the management cost.
  • TiCDC supports HTTP protocol OpenAPI to manage TiCDC tasks. It provides a more user-friendly operation method for both Kubernetes and self-hosted environments. (Experimental feature)

Compatibility changes

System variables

Variable nameChange typeDescription
default_authentication_pluginNewly addedSets the authentication method that the server advertises. The default value is mysql_native_password.
tidb_enable_auto_increment_in_generatedNewly addedDetermines whether to include the AUTO_INCREMENT columns when creating a generated column or an expression index. The default value is OFF.
tidb_opt_enable_correlation_adjustmentNewly addedControls whether the optimizer estimates the number of rows based on column order correlation. The default value is ON.
tidb_opt_limit_push_down_thresholdNewly addedSets the threshold that determines whether to push the Limit or TopN operator down to TiKV. The default value is 100.
tidb_stmt_summary_max_stmt_countModifiedSets the maximum number of statements that the statement summary tables store in memory. The default value is changed from 200 to 3000.
tidb_enable_streamingDeprecatedThe system variable enable-streaming is deprecated and it is not recommended to use it any more.

Configuration file parameters

Configuration fileConfiguration itemChange typeDescription
TiDB configuration filepessimistic-txn.deadlock-history-collect-retryableNewly addedControls whether the INFORMATION\_SCHEMA.DEADLOCKS table collects retryable deadlock error messages or not.
TiDB configuration addedDetermines whether to automatically generate the TLS certificates on startup. The default value is false.
TiDB configuration filestmt-summary.max-stmt-countModifiedIndicates the maximum number of SQL categories allowed to be saved in the statement summary tables. The default value is changed from 200 to 3000.
TiDB configuration fileexperimental.allow-expression-indexDeprecatedThe allow-expression-index configuration in the TiDB configuration file is deprecated.
TiKV configuration fileraftstore.cmd-batchNewly addedControls whether to enable batch processing of the requests. When it is enabled, the write performance is significantly improved. The default value is true.
TiKV configuration fileraftstore.inspect-intervalNewly addedAt a certain interval, TiKV inspects the latency of the Raftstore component. This configuration item specifies the interval of the inspection. If the latency exceeds this value, this inspection is marked as timeout. The default value is 500ms.
TiKV configuration fileraftstore.max-peer-down-durationModifiedIndicates the longest inactive duration allowed for a peer. A peer with timeout is marked as down, and PD tries to delete it later. The default value is changed from 5m to 10m.
TiKV configuration fileserver.raft-client-queue-sizeNewly addedSpecifies the queue size of the Raft messages in TiKV. The default value is 8192.
TiKV configuration filestorage.flow-control.enableNewly addedDetermines whether to enable the flow control mechanism. The default value is true.
TiKV configuration filestorage.flow-control.memtables-thresholdNewly addedWhen the number of kvDB memtables reaches this threshold, the flow control mechanism starts to work. The default value is 5.
TiKV configuration filestorage.flow-control.l0-files-thresholdNewly addedWhen the number of kvDB L0 files reaches this threshold, the flow control mechanism starts to work. The default value is 9.
TiKV configuration filestorage.flow-control.soft-pending-compaction-bytes-limitNewly addedWhen the pending compaction bytes in KvDB reach this threshold, the flow control mechanism starts to reject some write requests and reports the ServerIsBusy error. The default value is "192GB".
TiKV configuration filestorage.flow-control.hard-pending-compaction-bytes-limitNewly addedWhen the pending compaction bytes in KvDB reach this threshold, the flow control mechanism rejects all write requests and reports the ServerIsBusy error. The default value is "1024GB".


  • Before the upgrade, check whether the value of the tidb_evolve_plan_baselines system variable is ON. If the value is ON, set it to OFF; otherwise, the upgrade will fail.
  • For TiDB clusters upgraded from v4.0 to v5.2, the default value of tidb_multi_statement_mode changes from WARN to OFF.
  • Before the upgrade, check the value of the TiDB configuration feedback-probability. If the value is not 0, the "panic in the recoverable goroutine" error will occur after the upgrade, but this error does not affect the upgrade.
  • TiDB is now compatible with MySQL 5.7's noop variable innodb_default_row_format. Setting this variable has no effect. #23541
  • Starting from TiDB 5.2, to improve system security, it is recommended (but not mandatory) to encrypt the transport layer for connections from clients. TiDB provides the Auto TLS feature to automatically configure and enable encryption in TiDB. To use the Auto TLS feature, before the TiDB upgrade, set in the TiDB configuration file to true.
  • Support the caching_sha2_password authentication method to make migration from MySQL 8.0 easier and to improve security.

New features


  • Support using several functions in expression indexes

    The expression index is a type of special index that can be created on an expression. After an expression index is created, TiDB supports expression-based queries, which greatly improves query performance.

    User document, #25150

  • Support the translate function in Oracle

    The translate function replaces all occurrences of characters by other characters in a string. In TiDB, this function does not treat empty strings as NULL as Oracle does.

    User document

  • Support spilling HashAgg

    Support spilling HashAgg into disk. When a SQL statement that includes an HashAgg operator causes out of memory (OOM), you can try to set the concurrency of this operator to 1 to trigger disk spill, which alleviates memory stress.

    User document, #25882

  • Improve the accuracy of optimizer cardinality estimation

    • Improve the accuracy of TiDB's estimation of TopN/Limit. For example, for pagination queries on a large table that contain the order by col limit x condition, TiDB can more easily select the right index and reduce query response time.

    • Improve the accuracy of out-of-range estimation. For example, even if the statistics for a day have not been updated, TiDB can accurately select the corresponding index for a query that contains where date=Now().

    • Introduce the tidb_opt_limit_push_down_threshold variable to control the optimizer's behavior of pushing down Limit/TopN, which resolves the issue that Limit/TopN cannot be pushed down in some situations due to wrong estimation.

      User document, #26085

  • Improve index selection of the optimizer

    Add pruning rules for index selection. Before using the statistics for comparison, TiDB uses these rules to narrow down the scope of possible indexes to be selected, which reduces the possibility of selecting non-optimal indexes.

    User document


  • General availability (GA) for Lock View

    The Lock View feature provides more information about lock conflicts and lock waits of pessimistic locks, which helps DBAs to observe transaction locking events and troubleshoot deadlock problems.

    In v5.2, the following enhancements are made to Lock View:

    • In addition to the SQL digest column in the Lock View-related tables, add a column to these tables that shows the corresponding normalized SQL text. You do not have to manually query the statement corresponding to a SQL digest.

    • Add the TIDB_DECODE_SQL_DIGESTS function to query the normalized SQL statements (a form without formats and arguments) corresponding to a set of SQL digests in the cluster. This simplifies the operation of querying the statements that have been historically executed by a transaction.

    • Add a column in the DATA_LOCK_WAITS and DEADLOCKS system tables to show the table name, row ID, index value, and other key information interpreted from a key. This simplifies the operations such as locating the table to which a key belongs and interpreting the key information.

    • Support collecting the information of retryable deadlock errors in the DEADLOCKS table, which makes it easier to troubleshoot issues caused by such errors. The error collection is disabled by default and can be enabled using the pessimistic-txn.deadlock-history-collect-retryable configuration.

    • Support distinguishing query-executing transactions from idle transactions on the TIDB_TRX system table. The Normal state is now divided into Running and Idle states.

      User documents:

    • View the pessimistic lock-waiting events that are occurring on all TiKV nodes in the cluster: DATA_LOCK_WAITS

    • View the deadlock errors recently occurred on a TiDB node: DEADLOCKS

    • View the executing transaction on a TiDB node: TIDB_TRX

  • Optimize the user scenarios of adding indexes on tables with the AUTO_RANDOM or SHARD_ROW_ID_BITS attribute.


  • Add TiFlash I/O traffic limit

    This new feature is suitable for cloud storage with disk bandwidth of a small and specific size. It is disabled by default.

    TiFlash I/O Rate Limiter provides a new mechanism to avoid excessive race for I/O resources between read and write tasks. It balances the responses to read and write tasks, and limits the rate automatically according to read/write workload.

    User document

  • Improve stability of TiKV flow control

    TiKV introduces a new flow control mechanism to replace the previous RocksDB write stall mechanism. Compared with the write stall mechanism, this new mechanism reduces the impact on the stability of foreground write.

    In specific, when the stress of RocksDB compaction accumulates, flow control is performed at the TiKV scheduler layer instead of the RocksDB layer, to avoid the following issues:

    • Raftstore is stuck, which is caused by RocksDB write stall.

    • Raft election times out, and the node leader is transferred as a result.

      This new mechanism improves the flow control algorithm to mitigate QPS decrease when the write traffic is high.

      User document, #10137

  • Detect and recover automatically from impact caused by a single slow TiKV node in a cluster

    TiKV introduces the slow node detection mechanism. This mechanism calculates a score by inspecting the rate of TiKV Raftstore, and then reports the score to PD through store heartbeats. Meanwhile, it adds the evict-slow-store-scheduler scheduler on PD to automatically evict the leader on a single slow TiKV node. In this way, the impact on the whole cluster is mitigated. At the same time, more alert items about slow nodes are introduced to help you quickly pinpoint and solve problems.

    User document, #10539

Data Migration

  • Simplify operations of Data Migration (DM)

    DM v2.0.6 can automatically identify the change event (failover or plan change) of the data source using VIP, and can automatically connect to a new data source instance, to reduce data replication latency and simplify operation procedures.

  • TiDB Lightning supports customized line terminators in the CSV data, and is compatible with the MySQL LOAD DATA CSV data formats. You can then use TiDB Lightning directly in your data flow architecture.


TiDB data share subscription

TiCDC supports using the HTTP protocol (OpenAPI) to manage TiCDC tasks, which is a more user-friendly operation method for both Kubernetes and self-hosted environments. (Experimental feature)


Deployment and operations

Support running the tiup playground command on Mac computers with Apple M1 chips.

Feature Enhancements

  • Tools

    • TiCDC

      • Add the binary MQ format designed for TiDB. It is more compact than the open protocols based on JSON #1621
      • Remove support for file sorter #2114
      • Support log rotation configurations #2182
    • TiDB Lightning

      • Support customized line terminators (except \r and \n) #1297
      • Support expression index and the index that depends on virtual generated columns #1407
    • Dumpling

      • Support backing up MySQL compatible databases but does not support START TRANSACTION ... WITH CONSISTENT SNAPSHOT orSHOW CREATE TABLE #311


  • TiDB

    • Support pushing down the built-in function json_unquote() to TiKV #24415
    • Support removing the union branch from the dual table #25614
    • Optimize the aggregate operator's cost factor #25241
    • Allow the MPP outer join to choose the build table based on the table row count #25142
    • Support balancing the MPP query workload among different TiFlash nodes based on Regions #24724
    • Support invalidating stale Regions in the cache after the MPP query is executed #24432
    • Improve the MySQL compatibility of the built-in functionstr_to_date for the format specifiers %b/%M/%r/%T #25767
    • Fix the issue that inconsistent binding caches might be created in multiple TiDB after recreating different bindings for the same query #26015
    • Fix the issue that the existing bindings cannot be loaded into cache after upgrade #23295
    • Support ordering the result of SHOW BINDINGS by (original_sql, update_time) #26139
    • Improve the logic of query optimization when bindings exist, and reduce optimization times of a query #26141
    • Support completing the garbage collection automatically for the bindings in the "deleted" status #26206
    • Support showing whether a binding is used for query optimization in the result of EXPLAIN VERBOSE #26930
    • Add a new status variation last_plan_binding_update_time to view the timestamp corresponding to the binding cache in the current TiDB instance #26340
    • Support reporting an error when starting binding evolution or running admin evolve bindings to ban the baseline evolution (currently disabled in TiDB Self-Hosted because it is an experimental feature) affecting other features #26333
  • PD

    • Add more QPS dimensions for hot Region scheduling, and support adjusting the priority of the scheduling #3869
    • Support hot Region balance scheduling for the write hotspot of TiFlash #3900
  • TiFlash

    • Add operators: MOD / %, LIKE
    • Add string functions: ASCII(), COALESCE(), LENGTH(), POSITION(), TRIM()
    • Add mathematical functions: CONV(), CRC32(), DEGREES(), EXP(), LN(), LOG(), LOG10(), LOG2(), POW(), RADIANS(), ROUND(decimal), SIN(), MOD()
    • Add date functions: ADDDATE(string, real), DATE_ADD(string, real), DATE()
    • Add other functions: INET_NTOA(), INET_ATON(), INET6_ATON, INET6_NTOA()
    • Support Shuffled Hash Join calculation and Shuffled Hash Aggregation calculation in the MPP mode when a new collation is enabled
    • Optimize basic code to improve MPP performance
    • Support casting the STRING type to the DOUBLE type
    • Optimize the non-joined data in right outer join using multiple threads
    • Support automatically invalidating stale Regions in MPP queries
  • Tools

    • TiCDC

      • Add the concurrency limit to the incremental scan of kv client #1899
      • TiCDC can always pull the old value internally #2271
      • TiCDC can fail and exit fast when unrecoverable DML errors occur #1928
      • resolve lock cannot be run immediately after a Region is initialized #2235
      • Optimize workerpool to reduce the number of goroutines under high concurrency #2201
    • Dumpling

      • Support always splitting TiDB v3.x tables through tidb_rowid to save TiDB memory #301
      • Reduce access of Dumpling to the information_schema to improve stability #305

Bug Fixes

  • TiDB

    • Fix the issue that an incorrect result is returned when using merge join on the SET type column #25669
    • Fix the data corruption issue in the IN expression's arguments #25591
    • Avoid the sessions of GC being affected by global variables #24976
    • Fix the panic issue that occurs when using limit in the window function queries #25344
    • Fix the wrong value returned when querying a partitioned table using Limit #24636
    • Fix the issue that IFNULL does not correctly take effect on the ENUM or SET type column #24944
    • Fix the wrong results caused by changing the count in the join subqueries to first_row #24865
    • Fix the query hang issue that occurs when ParallelApply is used under the TopN operator #24930
    • Fix the issue that more results than expected are returned when executing SQL statements using multi-column prefix indexes #24356
    • Fix the issue that the <=> operator cannot correctly take effect #24477
    • Fix the data race issue of the parallel Apply operator #23280
    • Fix the issue that the index out of range error is reported when sorting the IndexMerge results of the PartitionUnion operator #23919
    • Fix the issue that setting the tidb_snapshot variable to an unexpectedly large value might damage the transaction isolation #25680
    • Fix the issue that the ODBC-styled constant (for example, {d '2020-01-01'}) cannot be used as the expression #25531
    • Fix the issue that SELECT DISTINCT converted to Batch Get causes incorrect results #25320
    • Fix the issue that backing off queries from TiFlash to TiKV cannot be triggered #23665 #24421
    • Fix the index-out-of-range error that occurs when checking only_full_group_by #23839)
    • Fix the issue that the result of index join in correlated subqueries is wrong #25799
  • TiKV

    • Fix the wrong tikv_raftstore_hibernated_peer_state metric #10330
    • Fix the wrong arguments type of the json_unquote() function in the coprocessor #10176
    • Skip clearing callback during graceful shutdown to avoid breaking ACID in some cases #10353 #10307
    • Fix a bug that the read index is shared for replica reads on a Leader #10347
    • Fix the wrong function that casts DOUBLE to DOUBLE #25200
  • PD

    • Fix the issue that the expected scheduling cannot be generated due to scheduling conflicts among multiple schedulers #3807 #3778
  • TiFlash

    • Fix the issue that TiFlash keeps restarting because of the split failure
    • Fix the potential issue that TiFlash cannot delete the delta data
    • Fix a bug that TiFlash adds wrong padding for non-binary characters in the CAST function
    • Fix the issue of incorrect results when handling aggregation queries with complex GROUP BY columns
    • Fix the TiFlash panic issue that occurs under heavy write pressure
    • Fix the panic that occurs when the right jon key is not nullable and the left join key is nullable
    • Fix the potential issue that the read-index requests take a long time
    • Fix the panic issue that occurs when the read load is heavy
    • Fix the panic issue that might occur when the Date_Format function is called with the STRING type argument and NULL values
  • Tools

    • TiCDC

      • Fix a bug that TiCDC owner exits abnormally when refreshing the checkpoint #1902
      • Fix a bug that changefeed fails immediately after its successful creation #2113
      • Fix a bug that changefeed fails due to the invalid format of rules filter #1625
      • Fix the potential DDL loss issue when the TiCDC owner panics #1260
      • Fix the CLI compatibility issue with 4.0.x clusters on the default sort-engine option #2373
      • Fix a bug that changefeed might be reset unexpectedly when TiCDC gets the ErrSchemaStorageTableMiss error #2422
      • Fix a bug that changefeed cannot be removed when TiCDC gets the ErrGCTTLExceeded error #2391
      • Fix a bug that TiCDC fails to synchronize large tables to cdclog #1259 #2424
      • Fix a bug that multiple processors might write data to the same table when TiCDC is rescheduling the table #2230
    • Backup & Restore (BR)

      • Fix a bug that BR skips restoring all system tables during the restore #1197 #1201
      • Fix a bug that BR misses DDL operations when restoring cdclog #870
    • TiDB Lightning

      • Fix a bug that TiDB Lightning fails to parse the DECIMAL data type in Parquet file #1272
      • Fix a bug that TiDB Lightning reports the "Error 9007: Write conflict" error when restoring table schemas #1290
      • Fix a bug that TiDB Lightning fails to import data due to the overflow of int handle #1291
      • Fix a bug that TiDB Lightning might get a checksum mismatching error due to data loss in the local backend mode #1403
      • Fix the Lighting incompatibility issue with clustered index when TiDB Lightning is restoring table schemas #1362
    • Dumpling

      • Fix a bug that the data export fails because the Dumpling GC safepoint is set too late #290
      • Fix the Dumpling getting stuck issue when exporting table names from the upstream database in certain MySQL versions #322

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