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sql-statement-unlock-stats.md

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UNLOCK STATS
An overview of the usage of UNLOCK STATS for the TiDB database.

UNLOCK STATS

UNLOCK STATS is used to unlock the statistics of a table or tables.

Warning:

Locking statistics is an experimental feature for the current version. It is not recommended to use it in the production environment.

Synopsis

UnlockStatsStmt ::=
    'UNLOCK' 'STATS' TableNameList

TableNameList ::=
    TableName (',' TableName)*

TableName ::=
    Identifier ( '.' Identifier )?

Examples

Create table t, and insert data into it. When the statistics of table t are not locked, the ANALYZE statement can be successfully executed.

mysql> create table t(a int, b int);
Query OK, 0 rows affected (0.03 sec)

mysql> insert into t values (1,2), (3,4), (5,6), (7,8);
Query OK, 4 rows affected (0.00 sec)
Records: 4  Duplicates: 0  Warnings: 0

mysql> analyze table t;
Query OK, 0 rows affected, 1 warning (0.02 sec)

mysql> show warnings;
+-------+------+-----------------------------------------------------------------+
| Level | Code | Message                                                         |
+-------+------+-----------------------------------------------------------------+
| Note  | 1105 | Analyze use auto adjusted sample rate 1.000000 for table test.t |
+-------+------+-----------------------------------------------------------------+
1 row in set (0.00 sec)

Lock the statistics of table t and execute ANALYZE. From the output of SHOW STATS_LOCKED, you can see that the statistics of table t have been locked. The warning message shows that the ANALYZE statement has skipped table t.

mysql> lock stats t;
Query OK, 0 rows affected (0.00 sec)

mysql> show stats_locked;
+---------+------------+----------------+--------+
| Db_name | Table_name | Partition_name | Status |
+---------+------------+----------------+--------+
| test    | t          |                | locked |
+---------+------------+----------------+--------+
1 row in set (0.01 sec)

mysql> analyze table t;
Query OK, 0 rows affected, 2 warnings (0.00 sec)

mysql> show warnings;
+---------+------+-----------------------------------------------------------------+
| Level   | Code | Message                                                         |
+---------+------+-----------------------------------------------------------------+
| Note    | 1105 | Analyze use auto adjusted sample rate 1.000000 for table test.t |
| Warning | 1105 | skip analyze locked table: t                                    |
+---------+------+-----------------------------------------------------------------+
2 rows in set (0.00 sec)

Unlock the statistics of table t and ANALYZE can be successfully executed again.

mysql> unlock stats t;
Query OK, 0 rows affected (0.01 sec)

mysql> analyze table t;
Query OK, 0 rows affected, 1 warning (0.03 sec)

mysql> show warnings;
+-------+------+-----------------------------------------------------------------+
| Level | Code | Message                                                         |
+-------+------+-----------------------------------------------------------------+
| Note  | 1105 | Analyze use auto adjusted sample rate 1.000000 for table test.t |
+-------+------+-----------------------------------------------------------------+
1 row in set (0.00 sec)

MySQL compatibility

This statement is a TiDB extension to MySQL syntax.

See also