面试官请不要再问我mysql联合索引失效、有效情况了

版本：5.5.18.1

联合索引建立情况：

查询条件顺序和联合索引一样

EXPLAIN SELECT * FROM t_cmp_mission WHERE companyID = "" AND isFinish ="" AND missionType ="" AND creTm = "";

查询条件顺序和联合索引顺序完全相反

EXPLAIN SELECT * FROM t_cmp_mission WHERE creTm = "" AND missionType ="" AND isFinish =""  AND companyID = "" ;

查询条件中不包含索引的第一个

EXPLAIN SELECT * FROM t_cmp_mission WHERE isFinish ="4" and missionType ="4" AND creTm ="";

查询条件只有联合索引的第一个索引

EXPLAIN SELECT * FROM t_cmp_mission WHERE companyID = "44";

只有联合索引的非第一个索引

EXPLAIN SELECT * FROM t_cmp_mission WHERE isFinish = "44";

非第一个索引列like

EXPLAIN SELECT * FROM t_cmp_mission WHERE companyID = "44" AND isFinish ="4" and missionType ="4" and creTm LIKE '%000%';

第一个索引列like 且双边模糊匹配

EXPLAIN SELECT * FROM t_cmp_mission WHERE companyID LIKE '%000%' AND isFinish ="4" and missionType ="4" and creTm ="";

非第一个索引列like 且双边模糊匹配

EXPLAIN SELECT * FROM t_cmp_mission WHERE  companyID ="" AND isFinish LIKE '%aa%' AND missionType ="4" and creTm ="";

第一个索引列like 且右边模糊匹配

EXPLAIN SELECT * FROM t_cmp_mission WHERE companyID LIKE '000%' AND isFinish ="4" and missionType ="4" and creTm ="";

第一个索引列like 且左边模糊匹配

EXPLAIN SELECT * FROM t_cmp_mission WHERE companyID LIKE '%000' AND isFinish ="4" and missionType ="4" and creTm ="";

查询条件中包含 or

EXPLAIN SELECT * FROM t_cmp_mission WHERE companyID ="someID" OR isFinish ="4" and missionType ="4" and creTm ="";

联合索引遇上 where 1=1

EXPLAIN SELECT * FROM t_cmp_mission WHERE 1=1 AND companyID ="someID" AND isFinish ="4" and missionType ="4" and creTm ="";

总结

1. 联合索引所有成员均以and 出现时与顺序无关，都能生效
2. 条件语句中出现or 则联合索引无效
3. 条件语句中缺少排在第一位的索引字段，整个联合索引失效
4. 欲使联合索引要生效，排在第一位的索引为必须出现，位置不限
5. 使用like 不一定会导致索引失效，只有like 使用在第一位的索引为且左边做了模糊匹配才会使索引失效
6. 或许是因为mysql内部做了优化，where 1=1并不影响索引的使用，不会扫描全表