数据库|mysql教程
mysql,索引
数据库-mysql教程
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mysql视频教学栏目索引失效的情况。
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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orexplain 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: userCreate Table: CREATE TABLE `user` ( `id` int(10) NOT NULL AUTO_INCREMENT, `name` varchar(20) DEFAULT NULL, `age` int(10) DEFAULT , `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
