MySQL統計信息查詢慢問題分析

起因

在MySQL服務器運行mysqld_exporter后，發現數據庫的中活躍連接數暴增，而且都是來自于mysqld_exporter的慢查詢，語句如下：

SELECT
            TABLE_SCHEMA,
            TABLE_NAME,
            TABLE_TYPE,
            ifnull(ENGINE, 'NONE') as ENGINE,
            ifnull(VERSION, '0') as VERSION,
            ifnull(ROW_FORMAT, 'NONE') as ROW_FORMAT,
            ifnull(TABLE_ROWS, '0') as TABLE_ROWS,
            ifnull(DATA_LENGTH, '0') as DATA_LENGTH,
            ifnull(INDEX_LENGTH, '0') as INDEX_LENGTH,
            ifnull(DATA_FREE, '0') as DATA_FREE,
            ifnull(CREATE_OPTIONS, 'NONE') as CREATE_OPTIONS
          FROM information_schema.tables
          WHERE TABLE_SCHEMA = 'xxx';

分析

1.在該數據庫執行該語句，執行時間非常慢

102 rows in set (6.35 sec)

2.在該數據庫的從庫執行，結果卻完全不一樣

102 rows in set (0.01 sec)

3.這個時候就可以確定應該跟MySQL統計信息有關。

查看MySQL統計信息相關介紹：https://blog.csdn.net/n88Lpo/article/details/79144495

4.查看主從數據庫的參數，發現差異

##主庫
mysql> show variables like 'innodb_stats_on_metadata';
+--------------------------+-------+
| Variable_name            | Value |
+--------------------------+-------+
| innodb_stats_on_metadata | ON    |
+--------------------------+-------+
1 row in set (0.00 sec)

##從庫
mysql> show variables like 'innodb_stats_on_metadata';
+--------------------------+-------+
| Variable_name            | Value |
+--------------------------+-------+
| innodb_stats_on_metadata | OFF   |
+--------------------------+-------+
1 row in set (0.00 sec)

5.確定原因為每次查詢時都會對統計信息進行更新。

查看MySQL官方文檔 https://dev.mysql.com/doc/refman/5.7/en/innodb-statistics-estimation.html

解決

修改參數innodb_stats_on_metadata

mysql> set global innodb_stats_on_metadata=0;
Query OK, 0 rows affected (0.00 sec)