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;
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| 10437 | usr0321t9m9 | 10.242.232.50:51201 | oms | Execute | 1179 | Sending |
Sql为:
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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:
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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:
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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:
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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:
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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
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ALTER TABLE MODIFY COLUMN PRONAME VARCHAR (156); |
再进行执行计划分析:
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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:
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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:
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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:
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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:
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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;