一张图搞懂MySQL的索引失效

索引对于MySQL而言，是很是重要的篇章。索引知识点也巨多，要想掌握透彻，须要逐个知识点一一击破，今天来先来聊聊哪些状况下会致使索引失效。

图片总结版

全值匹配（索引最佳）

explain select * from user where name = 'zhangsan' and age = 20 and pos = 'cxy' and phone = '18730658760';

和索引顺序无关，MySQL底层的优化器会进行优化，调整索引的顺序
explain select * from user where name = 'zhangsan' and age = 20 and pos = 'cxy' and phone = '18730658760';

一、违反最左前缀法则

若是索引有多列，要遵照最左前缀法则
即查询从索引的最左前列开始而且不跳过索引中的列
explain select * from user where age = 20 and phone = '18730658760' and pos = 'cxy';

二、在索引列上作任何操做

如计算、函数、（自动or手动）类型转换等操做，会致使索引失效从而全表扫描
explain select * from user where left(name,5) = 'zhangsan' and age = 20 and phone = '18730658760';

三、索引范围条件右边的列

索引范围条件右边的索引列会失效
explain select * from user where name = 'zhangsan' and age > 20 and pos = 'cxy';

四、尽可能使用覆盖索引

只访问索引查询（索引列和查询列一致），减小select*
explain select name,age,pos,phone from user where age = 20;

五、使用不等于（!=、<>）

mysql在使用不等于（!=、<>）的时候没法使用索引会致使全表扫描（除覆盖索引外）
explain select * from user where age != 20;
explain select * from user where age <> 20;

六、like以通配符开头（'%abc'）

索引失效
explain select * from user where name like '%zhangsan';

索引生效
explain select * from user where name like 'zhangsan%';

七、字符串不加单引号索引失效

explain select * from user where name = 2000;

八、or链接

少用or
explain select * from user where name = '2000' or age = 20 or pos ='cxy';

九、order by

正常（索引参与了排序）
explain select * from user where name = 'zhangsan' and age = 20 order by age,pos;
备注：索引有两个做用：排序和查找

致使额外的文件排序（会下降性能）
explain select name,age from user where name = 'zhangsan' order by pos;//违反最左前缀法则
explain select name,age from user where name = 'zhangsan' order by pos,age;//违反最左前缀法则
explain select * from user where name = 'zhangsan' and age = 20 order by created_time,age;//含非索引字段

十、group by

正常（索引参与了排序）
explain select name,age from user where name = 'zhangsan' group by age;
备注：分组以前必排序（排序同order by）

致使产生临时表（会下降性能）
explain select name,pos from user where name = 'zhangsan' group by pos;//违反最左前缀法则
explain select name,age from user where name = 'zhangsan' group by pos,age;//违反最左前缀法则
explain select name,age from user where name = 'zhangsan' group by age,created_time;//含非索引字段

使用的示例数据

mysql> show create table user \G
******************************************************
       Table: user
Create Table: CREATE TABLE `user` (
  `id` int(10) NOT NULL AUTO_INCREMENT,
  `name` varchar(20) DEFAULT NULL,
  `age` int(10) DEFAULT '0',
  `pos` varchar(30) DEFAULT NULL,
  `phone` varchar(11) DEFAULT NULL,
  `created_time` datetime DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `idx_name_age_pos_phone` (`name`,`age`,`pos`,`phone`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci