本文摘自PHP中文网，作者coldplay.xixi，侵删。
mysql视频教程栏目索引失效的情况。
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
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全值匹配（索引最佳）
1 | explain select * from user where name = 'zhangsan' and age = 20 and pos = 'cxy' and phone = '18730658760';
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1 2 | 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
1 2 3 | 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
1 2 | 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';
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3、索引范围条件右边的列
1 2 | 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';
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4、尽量使用覆盖索引
1 2 | 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;
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5、使用不等于（!=、<>）
1 2 3 | 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;
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6、like以通配符开头（'%abc'）
1 2 | 索引失效
explain select * from user where name like '%zhangsan';
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1 2 | 索引生效
explain select * from user where name like 'zhangsan%';
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7、字符串不加单引号索引失效
1 | explain select * from user where name = 2000;
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8、or连接
1 2 | 少用or
explain select * from user where name = '2000' or age = 20 or pos ='cxy';
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9、order by
1 2 3 | 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
|
1 2 3 4 | 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;
explain select name,age from user where name = 'zhangsan' order by pos,age;
explain select * from user where name = 'zhangsan' and age = 20 order by created_time,age;
|
10、group by
1 2 3 | 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）
|
1 2 3 4 | 导致产生临时表（会降低性能）
explain select name,pos from user where name = 'zhangsan' group by pos;
explain select name,age from user where name = 'zhangsan' group by pos,age;
explain select name,age from user where name = 'zhangsan' group by age,created_time;
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使用的示例数据
1 2 3 4 5 6 7 8 9 10 11 12 13 | 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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