sql-statements: add SHOW STATS_HEALTHY (#3388)

* sql-statements: ADD SHOW STATS_HEALTHY

* Update sql-statements/sql-statement-show-stats-healthy.md

Co-authored-by: Keke Yi <40977455+yikeke@users.noreply.github.com>

* Update sql-statements/sql-statement-show-stats-healthy.md

Co-authored-by: Keke Yi <40977455+yikeke@users.noreply.github.com>

Co-authored-by: Keke Yi <40977455+yikeke@users.noreply.github.com>
Co-authored-by: ti-srebot <66930949+ti-srebot@users.noreply.github.com>

TOC.md

@@ -325,6 +325,7 @@
       + [`SHOW PROFILES`](/sql-statements/sql-statement-show-profiles.md)
       + [`SHOW PUMP STATUS`](/sql-statements/sql-statement-show-pump-status.md)
       + [`SHOW SCHEMAS`](/sql-statements/sql-statement-show-schemas.md)
+      + [`SHOW STATS_HEALTHY`](/sql-statements/sql-statement-show-stats-healthy.md)
       + [`SHOW STATS_HISTOGRAMS`](/sql-statements/sql-statement-show-histograms.md)
       + [`SHOW STATES_META`](/sql-statements/sql-statement-show-stats-meta.md)
       + [`SHOW STATUS`](/sql-statements/sql-statement-show-status.md)

sql-statements/sql-statement-show-stats-healthy.md

@@ -0,0 +1,89 @@
+---
+title: SHOW STATS_HEALTHY
+summary: 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` table command. `ANALYZE` runs automatically when the health drops below the [`tidb_auto_analyze_ratio`](/system-variables.md#tidb_auto_analyze_ratio) threshold.
+
+## Synopsis
+
+**ShowStmt**
+
+![ShowStmt](/media/sqlgram/ShowStmt.png)
+
+**ShowTargetFiltertable**
+
+![ShowTargetFilterable](/media/sqlgram/ShowTargetFilterable.png)
+
+**ShowLikeOrWhereOpt**
+
+![ShowLikeOrWhereOpt](/media/sqlgram/ShowLikeOrWhereOpt.png)
+
+## Examples
+
+Load example data and run `ANALYZE`:
+
+{{< copyable "sql" >}}
+
+```sql
+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
+```
+
+```sql
+...
+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:
+
+{{< copyable "sql" >}}
+
+```sql
+DELETE FROM t1 WHERE id BETWEEN 101010 AND 201010; # delete about 30% of records
+SHOW STATS_HEALTHY; 
+```
+
+```sql
+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
+
+* [ANALYZE](/sql-statements/sql-statement-analyze-table.md)
+* [Introduction to Statistics](/statistics.md)