浅谈MySQL中的子查询优化技巧
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：
selecttradedto0_.*froma1tradedto0_wheretradedto0_.tradestatus='1' and(tradedto0_.tradeoidin(selectorderdto1_.tradeoidfroma2orderdto1_where orderdto1_.pronamelike'%??%'ororderdto1_.procodelike'%??%'))andtradedto0_.undefine4='1' andtradedto0_.invoicetype='1'andtradedto0_.tradestep='0'and(tradedto0_.orderCompanylike'0002%')orderbytradedto0_.tradesignASC,tradedto0_.makertimedesclimit15;
2.其他表的更新被阻塞：
updatea1settradesign='DAB67634-795C-4EAC-B4A0-78F0D531D62F',
markColor='#CD5555',memotime='2012-09-22',markPerson='??'wheretradeoidin('gy2012092204495100032')；
为了尽快恢复应用，将其长时间执行的sqlkill掉后，应用恢复正常;
3.分析执行计划:
db@3306：explainselecttradedto0_.*froma1tradedto0_wheretradedto0_.tradestatus='1'and(tradedto0_.tradeoidin(selectorderdto1_.tradeoid froma2orderdto1_whereorderdto1_.pronamelike'%??%'ororderdto1_.procodelike'%??%'))andtradedto0_.undefine4='1'andtradedto0_.invoicetype='1'andtradedto0_.tradestep='0'and(tradedto0_.orderCompanylike'0002%')orderbytradedto0_.tradesignASC,tradedto0_.makertimedesclimit15; +----+--------------------+------------+------+---------------+------+---------+------+-------+----- |id|select_type|table|type|possible_keys|key|key_len|ref|rows|Extra| +----+--------------------+------------+------+---------------+------+---------+------+-------+----- |1|PRIMARY|tradedto0_|ALL|NULL|NULL|NULL|NULL|27454|Usingwhere;Usingfilesort| |2|DEPENDENTSUBQUERY|orderdto1_|ALL|NULL|NULL|NULL|NULL|40998|Usingwhere| +----+--------------------+------------+------+---------------+------+---------+------+-------+-----
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：altertablea2addindexind_a2(proname,procode,tradeoid); ERROR1071(42000):Specifiedkeywastoolong;maxkeylengthis1000bytes
添加组合索引超过了最大keylength限制：
B．查看该表的字段定义：
db@3306：DESCa2; +---------------------+---------------+------+-----+---------+-------+ |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：SELECTMAX(LENGTH(PRONAME)),avg(LENGTH(PRONAME))FROMa2; +----------------------+----------------------+ |MAX(LENGTH(PRONAME))|avg(LENGTH(PRONAME))| +----------------------+----------------------+ |95|24.5588|
D．缩小字段长度
ALTERTABLEMODIFYCOLUMNPRONAMEVARCHAR(156);
再进行执行计划分析：
db@3306：explainselecttradedto0_.*froma1tradedto0_wheretradedto0_.tradestatus='1'and(tradedto0_.tradeoidin(selectorderdto1_.tradeoidfroma2orderdto1_whereorderdto1_.pronamelike'%??%'ororderdto1_.procodelike'%??%'))andtradedto0_.undefine4='1'andtradedto0_.invoicetype='1'andtradedto0_.tradestep='0'and(tradedto0_.orderCompanylike'0002%')orderbytradedto0_.tradesignASC,tradedto0_.makertimedesclimit15; +----+--------------------+------------+-------+-----------------+----------------------+---------+ |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|Usingwhere;Usingfilesort| |2|DEPENDENTSUBQUERY|orderdto1_|index|NULL|ind_a2|777|NULL|41005|Usingwhere;Usingindex| +----+--------------------+------------+-------+-----------------+----------------------+---------+
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：selectorderdto1_.tradeoidfromtorderdto1_whereorderdto1_.pronamelike'%??%'ororderdto1_.procodelike'%??%'; Emptyset(0.05sec)
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：
selecttradedto0_.*froma1tradedto0_,(selectorderdto1_.tradeoidfroma2orderdto1_whereorderdto1_.pronamelike'%??%'ororderdto1_.procodelike'%??%')t2wheretradedto0_.tradestatus='1'and(tradedto0_.tradeoid=t2.tradeoid)andtradedto0_.undefine4='1'andtradedto0_.invoicetype='1'andtradedto0_.tradestep='0'and(tradedto0_.orderCompanylike'0002%')orderbytradedto0_.tradesignASC,tradedto0_.makertimedesclimit15;
5.查看执行计划：
db@3306：explainselecttradedto0_.*froma1tradedto0_,(selectorderdto1_.tradeoidfroma2orderdto1_whereorderdto1_.pronamelike'%??%'ororderdto1_.procodelike'%??%')t2wheretradedto0_.tradestatus='1'and(tradedto0_.tradeoid=t2.tradeoid)andtradedto0_.undefine4='1'andtradedto0_.invoicetype='1'andtradedto0_.tradestep='0'and(tradedto0_.orderCompanylike'0002%')orderbytradedto0_.tradesignASC,tradedto0_.makertimedesclimit15; +----+-------------+------------+-------+---------------+----------------------+---------+------+ |id|select_type|table|type|possible_keys|key|key_len|ref|rows|Extra| +----+-------------+------------+-------+---------------+----------------------+---------+------+ |1|PRIMARY|NULL|NULL|NULL|NULL|NULL|NULL|NULL|ImpossibleWHEREnoticedafterreadingconsttables| |2|DERIVED|orderdto1_|index|NULL|ind_a2|777|NULL|41005|Usingwhere;Usingindex| +----+-------------+------------+-------+---------------+----------------------+---------+------+
6.执行时间：
db@3306：selecttradedto0_.*froma1tradedto0_,(selectorderdto1_.tradeoidfroma2orderdto1_whereorderdto1_.pronamelike'%??%'ororderdto1_.procodelike'%??%')t2wheretradedto0_.tradestatus='1'and(tradedto0_.tradeoid=t2.tradeoid)andtradedto0_.undefine4='1'andtradedto0_.invoicetype='1'andtradedto0_.tradestep='0'and(tradedto0_.orderCompanylike'0002%')orderbytradedto0_.tradesignASC,tradedto0_.makertimedesclimit15; Emptyset(0.03sec)
缩短到了毫秒；