MySQL一次大量内存消耗的跟踪

• GreatSQL社区原创内容未经授权不得随意使用，转载请联系小编并注明来源。
• GreatSQL是MySQL的国产分支版本，使用上与MySQL一致。
• 文章来源：GreatSQL社区原创

线上使用MySQL8.0.25的数据库，通过监控发现数据库在查询一个视图(80张表的union all)时内存和cpu均明显上升。

在8.0.25 MySQL Community Server官方版本测试发现：只能在视图上进行数据过滤，不能将视图上的过滤条件下推到视图内的表上进行数据过滤。8.0.29以后的版本已解决该问题。

MySQL视图访问原理

下面是在8.0.25 MySQL Community Server上做的测试

使用sysbench 构造4张1000000的表

 mysql> select count(*) from sbtest1;+----------+| count(*) |+----------+|  1000000 |+----------+1 row in set (1.44 sec)mysql> show create table sbtest1;| Table   | Create Table  | sbtest1 | CREATE TABLE `sbtest1` (  `id` int NOT NULL AUTO_INCREMENT,  `k` int NOT NULL DEFAULT '0',  `c` char(120) COLLATE utf8mb4_0900_bin NOT NULL DEFAULT '',  `pad` char(60) COLLATE utf8mb4_0900_bin NOT NULL DEFAULT '',  PRIMARY KEY (`id`)) ENGINE=InnoDB AUTO_INCREMENT=2000000 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_bin |+---------+-----------------------------------------------------------------------------------1 row in set (0.00 sec)

手工收集表统计信息

mysql> analyze table sbtest1,sbtest2 ,sbtest3,sbtest4;+----------------+---------+----------+----------+| Table          | Op      | Msg_type | Msg_text |+----------------+---------+----------+----------+| sbtest.sbtest1 | analyze | status   | OK       || sbtest.sbtest2 | analyze | status   | OK       || sbtest.sbtest3 | analyze | status   | OK       || sbtest.sbtest4 | analyze | status   | OK       |+----------------+---------+----------+----------+4 rows in set (0.17 sec)

创建视图

drop view view_sbtest1 ;Create view view_sbtest1  as select * from sbtest1 union all select * from sbtest2 union all select * from sbtest3 union all select * from sbtest4;

查询视图

Select * from view_sbtest1 where id=1; mysql> Select id ,k,left(c,20) from view_sbtest1 where id=1;+----+--------+----------------------+| id | k      | left(c,20)           |+----+--------+----------------------+|  1 | 434041 | 61753673565-14739672 ||  1 | 501130 | 64733237507-56788752 ||  1 | 501462 | 68487932199-96439406 ||  1 | 503019 | 18034632456-32298647 |+----+--------+----------------------+4 rows in set (1 min 8.96 sec)

通过主键查询数据， 查询返回4条数据，耗时1分8.96秒查看执行计划

从执行计划上看，先对视图内的表进行全表扫描，最后在视图上过滤数据。

mysql> explain Select id ,k,left(c,20) from view_sbtest1 where id=1;+----+-------------+------------+------------+------+---------------+-------------+---------+-------+--------+----------+-------+| id | select_type | table      | partitions | type | possible_keys | key         | key_len | ref   | rows   | filtered | Extra |+----+-------------+------------+------------+------+---------------+-------------+---------+-------+--------+----------+-------+|  1 | PRIMARY     |  | NULL       | ref  |    |  | 4       | const |     10 |   100.00 | NULL  ||  2 | DERIVED     | sbtest1    | NULL       | ALL  | NULL          | NULL        | NULL    | NULL  | 986400 |   100.00 | NULL  ||  3 | UNION       | sbtest2    | NULL       | ALL  | NULL          | NULL        | NULL    | NULL  | 986400 |   100.00 | NULL  ||  4 | UNION       | sbtest3    | NULL       | ALL  | NULL          | NULL        | NULL    | NULL  | 986400 |   100.00 | NULL  ||  5 | UNION       | sbtest4    | NULL       | ALL  | NULL          | NULL        | NULL    | NULL  | 986400 |   100.00 | NULL  |+----+-------------+------------+------------+------+---------------+-------------+---------+-------+--------+----------+-------+5 rows in set, 1 warning (0.07 sec)  

添加hint后的执行计划

添加官方的 merge hint 进行视图合并(期望视图不作为一个整体，让where上的过滤条件能下推到视图中的表)，不能改变sql执行计划，优化器需要先进行全表扫描在对结果集进行过滤。sql语句的执行时间基本不变

mysql> explain Select /*+  merge(t1) */ id ,k,left(c,20) from view_sbtest1 t1 where id=1;+----+-------------+------------+------------+------+---------------+-------------+---------+-------+--------+----------+-------+| id | select_type | table      | partitions | type | possible_keys | key         | key_len | ref   | rows   | filtered | Extra |+----+-------------+------------+------------+------+---------------+-------------+---------+-------+--------+----------+-------+|  1 | PRIMARY     |  | NULL       | ref  |    |  | 4       | const |     10 |   100.00 | NULL  ||  2 | DERIVED     | sbtest1    | NULL       | ALL  | NULL          | NULL        | NULL    | NULL  | 986400 |   100.00 | NULL  ||  3 | UNION       | sbtest2    | NULL       | ALL  | NULL          | NULL        | NULL    | NULL  | 986400 |   100.00 | NULL  ||  4 | UNION       | sbtest3    | NULL       | ALL  | NULL          | NULL        | NULL    | NULL  | 986400 |   100.00 | NULL  ||  5 | UNION       | sbtest4    | NULL       | ALL  | NULL          | NULL        | NULL    | NULL  | 986400 |   100.00 | NULL  |+----+-------------+------------+------------+------+---------------+-------------+---------+-------+--------+----------+-------+5 rows in set, 1 warning (0.00 sec)

创建视图(过滤条件在视图内)

mysql> drop view view_sbtest3;ERROR 1051 (42S02): Unknown table 'sbtest.view_sbtest3'mysql> Create view view_sbtest3 as      select * from sbtest1 where id=1      union all       select * from sbtest2 where id=1     union all     select * from sbtest3  where id=1     union all     select * from sbtest4 where id=1;Query OK, 0 rows affected (0.02 sec)

查询视图(过滤条件在视图上)

Select id ,k,left(c,20) from view_sbtest3 where id=1;mysql>  Select id ,k,left(c,20) from view_sbtest3 where id=1;+----+--------+----------------------+| id | k      | left(c,20)           |+----+--------+----------------------+|  1 | 501462 | 68487932199-96439406 ||  1 | 434041 | 61753673565-14739672 ||  1 | 501130 | 64733237507-56788752 ||  1 | 503019 | 18034632456-32298647 |+----+--------+----------------------+4 rows in set (0.01 sec)

直接运行sql语句

 mysql> select id ,k,left(c,20) from sbtest1 where id=1      ->  union all     ->  select id ,k,left(c,20) from sbtest2 where id=1      ->  union all     ->  select id ,k,left(c,20) from sbtest3 where id=1     ->  union all     ->  select id ,k,left(c,20) from sbtest4 where id=1;+----+--------+----------------------+| id | k      | left(c,20)           |+----+--------+----------------------+|  1 | 501462 | 68487932199-96439406 ||  1 | 434041 | 61753673565-14739672 ||  1 | 501130 | 64733237507-56788752 ||  1 | 503019 | 18034632456-32298647 |+----+--------+----------------------+4 rows in set (0.01 sec)

直接运行sql语句或者把过滤条件放到视图内均能很快得到数据。

8.0.32

新的MySQL8.0.32版本 已解决掉该问题，视图上的过滤条件能下推到表上。

 Server version: 8.0.32 MySQL Community Server - GPLCopyright (c) 2000, 2023, Oracle and/or its affiliates.Oracle is a registered trademark of Oracle Corporation and/or itsaffiliates. Other names may be trademarks of their respectiveowners.Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.mysql> use sbtest;Reading table information for completion of table and column namesYou can turn off this feature to get a quicker startup with -ADatabase changedmysql> Select id ,k,left(c,20) from view_sbtest1 where id=1;+----+--------+----------------------+| id | k      | left(c,20)           |+----+--------+----------------------+|  1 | 501462 | 68487932199-96439406 ||  1 | 434041 | 61753673565-14739672 ||  1 | 501130 | 64733237507-56788752 ||  1 | 503019 | 18034632456-32298647 |+----+--------+----------------------+4 rows in set (0.01 sec)mysql> Select id ,k,left(c,20) from view_sbtest3 where id=1;+----+--------+----------------------+| id | k      | left(c,20)           |+----+--------+----------------------+|  1 | 501462 | 68487932199-96439406 ||  1 | 434041 | 61753673565-14739672 ||  1 | 501130 | 64733237507-56788752 ||  1 | 503019 | 18034632456-32298647 |+----+--------+----------------------+4 rows in set (0.00 sec)

Enjoy GreatSQL ?

关于 GreatSQL

GreatSQL是由万里数据库维护的MySQL分支，专注于提升MGR可靠性及性能，支持InnoDB并行查询特性，是适用于金融级应用的MySQL分支版本。

相关链接： GreatSQL社区GiteeGitHubBilibili

GreatSQL社区：

社区博客有奖征稿详情：https://greatsql.cn/thread-100-1-1.html

技术交流群：

微信：扫码添加GreatSQL社区助手微信好友，发送验证信息加群。