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。
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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编辑mysql