介绍mysql索引失效的状况

全值匹配（索引最好）

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

和索引次序无关，MySQL底层的优化器会停止优化，调整索引的次序
explain select * from user where name = 'zhangsan' and age = 20 and pos = 'cxy' and phone = '18730658760';

1、违反最左前缀规则

假如索引有多列，要遵照最左前缀规则
即查询从索引的最左前列开端并且不跳过索引中的列
explain select * from user where age = 20 and phone = '18730658760' and pos = 'cxy';

2、在索引列上做任何操纵

如运算、函数、（主动or手动）类型转换等操纵，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';

3、索引范畴前提右侧的列

索引范畴前提右侧的索引列会失效
explain select * from user where name = 'zhangsan' and age > 20 and pos = 'cxy';

4、尽量使用覆盖索引

只拜访索引查询（索引列和查询列一致），减少select*
explain select name,age,pos,phone from user where age = 20;

5、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;

6、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%';

7、字符串不加单引号索引失效

explain select * from user where name = 2000;

8、or连接

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

9、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：索引有两个作用：排序和查寻

致使额外的文件排序（会落低机能）
explain select name,age from user where name = 'zhangsan' order by pos;//违反最左前缀规则
explain select name,age from user where name = 'zhangsan' order by pos,age;//违反最左前缀规则
explain select * from user where name = 'zhangsan' and age = 20 order by created_time,age;//含非索引字段

10、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：分组此前必排序（排序同order by）

致使发生暂时表（会落低机能）
explain select name,pos from user where name = 'zhangsan' group by pos;//违反最左前缀规则
explain select name,age from user where name = 'zhangsan' group by pos,age;//违反最左前缀规则
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

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