To enhance read performance and reduce disk usage, TiDB automatically schedules data compaction on storage nodes in the background. During the compaction, storage nodes rewrite physical data, including cleaning up deleted rows and merging multiple versions of data caused by updates. The
ALTER TABLE ... COMPACT statement allows you to initiate compaction for a specific table immediately, without waiting until compaction is triggered in the background.
The execution of this statement does not block existing SQL statements or affect any TiDB features, such as transactions, DDL, and GC. Data that can be selected via SQL statements will not be changed either. Executing this statement consumes some IO and CPU resources. Be careful to choose an appropriate timing for execution, such as when resources are available, to avoid negative impact on the business.
The compaction statement will be finished and returned when all replicas of a table are compacted. During the execution process, you can safely interrupt the compaction by executing the
KILL statement. Interrupting a compaction does not break data consistency or lead to data loss, nor does it affect subsequent manual or background compactions.
This data compaction statement is currently supported only for TiFlash replicas, not for TiKV replicas.
AlterTableCompactStmt ::= 'ALTER' 'TABLE' TableName 'COMPACT' ( 'TIFLASH' 'REPLICA' )?
Since v6.2.0, the
TIFLASH REPLICA part of the syntax can be omitted. When omitted, the semantic of the statement remains unchanged, and takes effect only for TiFlash.
The following takes an
employees table as an example, which has 4 partitions with 2 TiFlash replicas:
CREATE TABLE employees ( id INT NOT NULL, hired DATE NOT NULL DEFAULT '1970-01-01', store_id INT ) PARTITION BY LIST (store_id) ( PARTITION pNorth VALUES IN (1, 2, 3, 4, 5), PARTITION pEast VALUES IN (6, 7, 8, 9, 10), PARTITION pWest VALUES IN (11, 12, 13, 14, 15), PARTITION pCentral VALUES IN (16, 17, 18, 19, 20) ); ALTER TABLE employees SET TIFLASH REPLICA 2;
You can execute the following statement to immediately initiate the compaction for the 2 TiFlash replicas for all partitions in the
ALTER TABLE employees COMPACT TIFLASH REPLICA;
ALTER TABLE ... COMPACT statement compacts all replicas in a table simultaneously.
To avoid a significant impact on online business, each TiFlash instance only compacts data in one table at a time by default (except for the compaction triggered in the background). This means that if you execute the
ALTER TABLE ... COMPACT statement on multiple tables at the same time, their executions will be queued on the same TiFlash instance, rather than being executed simultaneously.
To obtain greater table-level concurrency with higher resource usage, you can modify the TiFlash configuration
manual_compact_pool_size. For example, when
manual_compact_pool_size is set to 2, compaction for 2 tables can be processed simultaneously.
ALTER TABLE ... COMPACT syntax is TiDB specific, which is an extension to the standard SQL syntax. Although there is no equivalent MySQL syntax, you can still execute this statement by using MySQL clients or various database drivers that comply with the MySQL protocol.
ALTER TABLE ... COMPACT statement does not result in logical data changes and are therefore not replicated to downstream by TiDB Binlog or TiCDC.