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 表命令来改善表的健康度。当表的健康度下降到低于 tidb_auto_analyze_ratio 时，则会自动执行 ANALYZE 命令。

语法图

ShowStmt

ShowStmt

ShowTargetFilterable

ShowTargetFilterable

ShowLikeOrWhereOpt

ShowLikeOrWhereOpt

示例

加载示例数据并运行 ANALYZE 命令：

1. CREATE TABLE t1 (
2. id INT NOT NULL PRIMARY KEY auto_increment,
3. b INT NOT NULL,
4. pad VARBINARY(255),
5. INDEX(b)
6. );
7. INSERT INTO t1 SELECT NULL, FLOOR(RAND()*1000), RANDOM_BYTES(255) FROM dual;
8. INSERT INTO t1 SELECT NULL, FLOOR(RAND()*1000), RANDOM_BYTES(255) FROM t1 a JOIN t1 b JOIN t1 c LIMIT 100000;
9. INSERT INTO t1 SELECT NULL, FLOOR(RAND()*1000), RANDOM_BYTES(255) FROM t1 a JOIN t1 b JOIN t1 c LIMIT 100000;
10. INSERT INTO t1 SELECT NULL, FLOOR(RAND()*1000), RANDOM_BYTES(255) FROM t1 a JOIN t1 b JOIN t1 c LIMIT 100000;
11. INSERT INTO t1 SELECT NULL, FLOOR(RAND()*1000), RANDOM_BYTES(255) FROM t1 a JOIN t1 b JOIN t1 c LIMIT 100000;
12. INSERT INTO t1 SELECT NULL, FLOOR(RAND()*1000), RANDOM_BYTES(255) FROM t1 a JOIN t1 b JOIN t1 c LIMIT 100000;
13. INSERT INTO t1 SELECT NULL, FLOOR(RAND()*1000), RANDOM_BYTES(255) FROM t1 a JOIN t1 b JOIN t1 c LIMIT 100000;
14. SELECT SLEEP(1);
15. ANALYZE TABLE t1;
16. SHOW STATS_HEALTHY; # should be 100% healthy

1. SHOW STATS_HEALTHY;

1. +---------+------------+----------------+---------+
2. | Db_name | Table_name | Partition_name | Healthy |
3. +---------+------------+----------------+---------+
4. | test    | t1         |                |     100 |
5. +---------+------------+----------------+---------+
6. 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：

1. DELETE FROM t1 WHERE id BETWEEN 101010 AND 201010; # delete about 30% of records
2. SHOW STATS_HEALTHY;

1. SHOW STATS_HEALTHY;
2. +---------+------------+----------------+---------+
3. | Db_name | Table_name | Partition_name | Healthy |
4. +---------+------------+----------------+---------+
5. | test    | t1         |                |      50 |
6. +---------+------------+----------------+---------+
7. 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