mysql的子查询的优化一直不是很友好，一直有受业界批评比较多,也是我在sql优化中遇到过最多的问题之一，你可以点击这里 ，这里来获得一些信息，mysql在处理子查询的时候，会将子查询改写,通常情况下，我们希望由内到外，也就是先完成子查询的结果，然后在用子查询来驱动外查询的表，完成查询，但是恰恰相反，子查询不会先被执行；今天希望通过介绍一些实际的案例来加深对mysql子查询的理解：
案例：用户反馈数据库响应较慢，许多业务动更新被卡住；登录到数据库中观察，发现长时间执行的sql；
| 10437 | usr0321t9m9 | 10.242.232.50:51201 | oms | execute | 1179 | sending
sql为：
select tradedto0_.* from a1 tradedto0_ where tradedto0_.tradestatus='1' and (tradedto0_.tradeoid in (select orderdto1_.tradeoid from a2 orderdto1_ where orderdto1_.proname like '%??%' or orderdto1_.procode like '%??%')) and tradedto0_.undefine4='1' and tradedto0_.invoicetype='1' and tradedto0_.tradestep='0' and (tradedto0_.ordercompany like '0002%') order by tradedto0_.tradesign asc, tradedto0_.makertime desc limit 15;
2.其他表的更新被阻塞：
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update a1 set tradesign='dab67634-795c-4eac-b4a0-78f0d531d62f',
markcolor=' #cd5555', memotime='2012-09- 22', markperson='??' where tradeoid in ('gy2012092204495100032') ；
为了尽快恢复应用，将其长时间执行的sql kill掉后，应用恢复正常;
3.分析执行计划:
db@3306 ：explain select tradedto0_.* from a1 tradedto0_ where tradedto0_.tradestatus='1' and (tradedto0_.tradeoid in (select orderdto1_.tradeoid from a2 orderdto1_ where orderdto1_.proname like '%??%' or orderdto1_.procode like '%??%')) and tradedto0_.undefine4='1' and tradedto0_.invoicetype='1' and tradedto0_.tradestep='0' and (tradedto0_.ordercompany like '0002%') order by tradedto0_.tradesign asc, tradedto0_.makertime desc limit 15; +----+--------------------+------------+------+---------------+------+---------+------+-------+----- | id | select_type | table | type | possible_keys | key | key_len | ref | rows | extra | +----+--------------------+------------+------+---------------+------+---------+------+-------+----- | 1 | primary | tradedto0_ | all | null | null | null | null | 27454 | using where; using filesort | | 2 | dependent subquery | orderdto1_ | all | null | null | null | null | 40998 | using where | +----+--------------------+------------+------+---------------+------+---------+------+-------+-----
从执行计划上，我们开始一步一步地进行优化：
首先，我们看看执行计划的第二行，也就是子查询的那部分，orderdto1_进行了全表的扫描，我们看看能不能添加适当的索引：
a.使用覆盖索引:
db@3306：alter table a2 add index ind_a2(proname,procode,tradeoid); error 1071 (42000): specified key was too long; max key length is 1000 bytes
添加组合索引超过了最大key length限制：
b．查看该表的字段定义：
db@3306 ：desc a2 ; +---------------------+---------------+------+-----+---------+-------+ | field | type | null | key | default | extra | +---------------------+---------------+------+-----+---------+-------+ | oid | varchar(50) | no | pri | null | | | tradeoid | varchar(50) | yes | | null | | | procode | varchar(50) | yes | | null | | | proname | varchar(1000) | yes | | null | | | spctncode | varchar(200) | yes | | null | |
c．查看表字段的平均长度：
db@3306 ：select max(length(proname)),avg(length(proname)) from a2; +----------------------+----------------------+ | max(length(proname)) | avg(length(proname)) | +----------------------+----------------------+ | 95 | 24.5588 |
d．缩小字段长度
alter table modify column proname varchar(156);
再进行执行计划分析：
db@3306 ：explain select tradedto0_.* from a1 tradedto0_ where tradedto0_.tradestatus='1' and (tradedto0_.tradeoid in (select orderdto1_.tradeoid from a2 orderdto1_ where orderdto1_.proname like '%??%' or orderdto1_.procode like '%??%')) and tradedto0_.undefine4='1' and tradedto0_.invoicetype='1' and tradedto0_.tradestep='0' and (tradedto0_.ordercompany like '0002%') order by tradedto0_.tradesign asc, tradedto0_.makertime desc limit 15; +----+--------------------+------------+-------+-----------------+----------------------+---------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | extra | +----+--------------------+------------+-------+-----------------+----------------------+---------+ | 1 | primary | tradedto0_ | ref | ind_tradestatus | ind_tradestatus | 345 | const,const,const,const | 8962 | using where; using filesort | | 2 | dependent subquery | orderdto1_ | index | null | ind_a2 | 777 | null | 41005 | using where; using index | +----+--------------------+------------+-------+-----------------+----------------------+---------+
发现性能还是上不去，关键在两个表扫描的行数并没有减小（8962*41005），上面添加的索引没有太大的效果，现在查看t表的执行结果：
db@3306 ：select orderdto1_.tradeoid from t orderdto1_ where orderdto1_.proname like '%??%' or orderdto1_.procode like '%??%'; empty set (0.05 sec)
结果集为空，所以需要将t表的结果集做作为驱动表；
4．通过上面测试验证，普通的mysql子查询写法性能上是很差的，为mysql的子查询天然的弱点，需要将sql进行改写为关联的写法：
select tradedto0_.* from a1 tradedto0_ ,(select orderdto1_.tradeoid from a2 orderdto1_ where orderdto1_.proname like '%??%' or orderdto1_.procode like '%??%')t2 where tradedto0_.tradestatus='1' and (tradedto0_.tradeoid=t2.tradeoid ) and tradedto0_.undefine4='1' and tradedto0_.invoicetype='1' and tradedto0_.tradestep='0' and (tradedto0_.ordercompany like '0002%') order by tradedto0_.tradesign asc, tradedto0_.makertime desc limit 15;
5.查看执行计划：
db@3306 ：explain select tradedto0_.* from a1 tradedto0_ ,(select orderdto1_.tradeoid from a2 orderdto1_ where orderdto1_.proname like '%??%' or orderdto1_.procode like '%??%')t2 where tradedto0_.tradestatus='1' and (tradedto0_.tradeoid=t2.tradeoid ) and tradedto0_.undefine4='1' and tradedto0_.invoicetype='1' and tradedto0_.tradestep='0' and (tradedto0_.ordercompany like '0002%') order by tradedto0_.tradesign asc, tradedto0_.makertime desc limit 15; +----+-------------+------------+-------+---------------+----------------------+---------+------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | extra | +----+-------------+------------+-------+---------------+----------------------+---------+------+ | 1 | primary | null | null | null | null | null | null | null | impossible where noticed after reading const tables | | 2 | derived | orderdto1_ | index | null | ind_a2 | 777 | null | 41005 | using where; using index | +----+-------------+------------+-------+---------------+----------------------+---------+------+
6.执行时间：
db@3306 ：select tradedto0_.* from a1 tradedto0_ ,(select orderdto1_.tradeoid from a2 orderdto1_ where orderdto1_.proname like '%??%' or orderdto1_.procode like '%??%')t2 where tradedto0_.tradestatus='1' and (tradedto0_.tradeoid=t2.tradeoid ) and tradedto0_.undefine4='1' and tradedto0_.invoicetype='1' and tradedto0_.tradestep='0' and (tradedto0_.ordercompany like '0002%') order by tradedto0_.tradesign asc, tradedto0_.makertime desc limit 15; empty set (0.03 sec)
缩短到了毫秒；
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