Skip to content

Latest commit

 

History

History
91 lines (71 loc) · 3.06 KB

sql-statement-show-stats-healthy.md

File metadata and controls

91 lines (71 loc) · 3.06 KB
Raw
title summary
SHOW STATS_HEALTHY
An overview of the usage of SHOW STATS_HEALTHY for TiDB database.

SHOW STATS_HEALTHY

The SHOW STATS_HEALTHY statement shows an estimation of how accurate statistics are believed to be. Tables with a low percentage health may generate sub-optimal query execution plans.

The health of a table can be improved by running the ANALYZE statement. ANALYZE runs automatically when the health drops below the tidb_auto_analyze_ratio threshold.

Currently, the SHOW STATS_HEALTHY statement returns the following columns:

Column name Description
Db_name The database name
Table_name The table name
Partition_name The partition name
Healthy The healthy percentage between 0 and 100

Synopsis

ShowStatsHealthyStmt ::=
    "SHOW" "STATS_HEALTHY" ShowLikeOrWhere?

ShowLikeOrWhere ::=
    "LIKE" SimpleExpr
|   "WHERE" Expression

Examples

Load example data and run ANALYZE:

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
...
mysql> SHOW STATS_HEALTHY;
+---------+------------+----------------+---------+
| Db_name | Table_name | Partition_name | Healthy |
+---------+------------+----------------+---------+
| test    | t1         |                |     100 |
+---------+------------+----------------+---------+
1 row in set (0.00 sec)

Perform a bulk update deleting approximately 30% of the records. Check the health of the statistics:

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

MySQL compatibility

This statement is a TiDB extension to MySQL syntax.

See also