[一张图搞懂MySQL的索引失效]((https://segmentfault.com/a/1190000021464570)

全值匹配（索引最佳）

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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

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

1、违反最左前缀法则 如果索引有多列，要遵守最左前缀法则 即查询从索引的最左前列开始并且不跳过索引中的列

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

2、在索引列上做任何操作 如计算、函数、（自动or手动）类型转换等操作，会导致索引失效从而全表扫描

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explain select * from user where left(name,5) = 'zhangsan' and age = 20 and phone = '18730658760';

3、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

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

4、尽量使用覆盖索引 只访问索引查询（索引列和查询列一致），减少select*

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explain select name,age,pos,phone from user where age = 20;

5、使用不等于（!=、<>） mysql在使用不等于（!=、<>）的时候无法使用索引会导致全表扫描（除覆盖索引外）

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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

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

索引生效

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

7、字符串不加单引号索引失效

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explain select * from user where name = 2000;

8、or连接 少用or

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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）

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explain select * from user where name = 'zhangsan' and age = 20 order by age,pos;

备注：索引有两个作用：排序和查找

导致额外的文件排序（会降低性能）

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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

10、group by 正常（索引参与了排序）

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explain select name,age from user where name = 'zhangsan' group by age;

备注：分组之前必排序（排序同order by）

导致产生临时表（会降低性能）

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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

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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

参考