一张图搞懂MySQL的索引失效

Posted 2021-03-19 sun666

索引对于mysql而言，是非常重要的篇章。索引知识点也巨多，要想掌握透彻，需要逐个知识点一一击破，今天来先来聊聊哪些情况下会导致索引失效。 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\';

　　

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
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
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
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*
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）
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：排序和查找

　　

导致额外的文件排序（会降低性能）
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

　　

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：分组之前必排序（排序同order by）

　　

导致产生临时表（会降低性能）
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

　　

了解更多SQL常见问题及解决方式请查看原文：https://developer.aliyun.com/article/741403?utm_content=g_1000097870