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：
