SHOW STATS_HEALTHY

SHOW STATS_HEALTHY 语句可以预估统计信息的准确度,也就是健康度。健康度低的表可能会生成次优查询执行计划。

可以通过执行 ANALYZE 语句来改善表的健康度。当表的健康度下降到低于 tidb_auto_analyze_ratio 时,则会自动执行 ANALYZE 语句。

语法图

ShowStatsHealthyStmt
SHOWSTATS_HEALTHYShowLikeOrWhere
ShowLikeOrWhere
LIKESimpleExprWHEREExpression

示例

加载示例数据并运行 ANALYZE 语句:

CREATE TABLE t1 ( id INT NOT NULL PRIMARY KEY auto_increment, b INT NOT NULL, pad VARBINARY(255), INDEX(b) ); INSERT INTO t1 SELECT NULL, FLOOR(RAND()*1000), RANDOM_BYTES(255) FROM dual; INSERT INTO t1 SELECT NULL, FLOOR(RAND()*1000), RANDOM_BYTES(255) FROM t1 a JOIN t1 b JOIN t1 c LIMIT 100000; INSERT INTO t1 SELECT NULL, FLOOR(RAND()*1000), RANDOM_BYTES(255) FROM t1 a JOIN t1 b JOIN t1 c LIMIT 100000; INSERT INTO t1 SELECT NULL, FLOOR(RAND()*1000), RANDOM_BYTES(255) FROM t1 a JOIN t1 b JOIN t1 c LIMIT 100000; INSERT INTO t1 SELECT NULL, FLOOR(RAND()*1000), RANDOM_BYTES(255) FROM t1 a JOIN t1 b JOIN t1 c LIMIT 100000; INSERT INTO t1 SELECT NULL, FLOOR(RAND()*1000), RANDOM_BYTES(255) FROM t1 a JOIN t1 b JOIN t1 c LIMIT 100000; INSERT INTO t1 SELECT NULL, FLOOR(RAND()*1000), RANDOM_BYTES(255) FROM t1 a JOIN t1 b JOIN t1 c LIMIT 100000; SELECT SLEEP(1); ANALYZE TABLE t1; SHOW STATS_HEALTHY; # should be 100% healthy
SHOW STATS_HEALTHY;
+---------+------------+----------------+---------+ | Db_name | Table_name | Partition_name | Healthy | +---------+------------+----------------+---------+ | test | t1 | | 100 | +---------+------------+----------------+---------+ 1 row in set (0.00 sec)

执行批量更新来删除大约 30% 的记录,然后检查统计信息的健康度:

DELETE FROM t1 WHERE id BETWEEN 101010 AND 201010; # delete about 30% of records SHOW STATS_HEALTHY;
SHOW STATS_HEALTHY; +---------+------------+----------------+---------+ | Db_name | Table_name | Partition_name | Healthy | +---------+------------+----------------+---------+ | test | t1 | | 50 | +---------+------------+----------------+---------+ 1 row in set (0.00 sec)

MySQL 兼容性

SHOW STATS_HEALTHY 语句是 TiDB 对 MySQL 语法的扩展。

另请参阅

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