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 - GPL
Copyright (c) 2000, 2023, Oracle and/or its affiliates.
Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.
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 names
You can turn off this feature to get a quicker startup with -A
Database changed
mysql> 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)
