一張圖搞懂MySQL的索引失效

索引對於MySQL而言，是很是重要的篇章。索引知識點也巨多，要想掌握透徹，須要逐個知識點一一擊破，今天來先來聊聊哪些狀況下會致使索引失效。

圖片總結版

全值匹配（索引最佳）

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

和索引順序無關，MySQL底層的優化器會進行優化，調整索引的順序
explain select * from user where name = 'zhangsan' and age = 20 and pos = 'cxy' and phone = '18730658760';

一、違反最左前綴法則

若是索引有多列，要遵照最左前綴法則
即查詢從索引的最左前列開始而且不跳過索引中的列
explain select * from user where age = 20 and phone = '18730658760' and pos = 'cxy';

二、在索引列上作任何操做

如計算、函數、（自動or手動）類型轉換等操做，會致使索引失效從而全表掃描
explain select * from user where left(name,5) = 'zhangsan' and age = 20 and phone = '18730658760';

三、索引範圍條件右邊的列

索引範圍條件右邊的索引列會失效
explain select * from user where name = 'zhangsan' and age > 20 and pos = 'cxy';

四、儘可能使用覆蓋索引

只訪問索引查詢（索引列和查詢列一致），減小select*
explain select name,age,pos,phone from user where age = 20;

五、使用不等於（!=、<>）

mysql在使用不等於（!=、<>）的時候沒法使用索引會致使全表掃描（除覆蓋索引外）
explain select * from user where age != 20;
explain select * from user where age <> 20;

六、like以通配符開頭（'%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%';

七、字符串不加單引號索引失效

explain select * from user where name = 2000;

八、or鏈接

少用or
explain select * from user where name = '2000' or age = 20 or pos ='cxy';

九、order by

正常（索引參與了排序）
explain select * from user where name = 'zhangsan' and age = 20 order by age,pos;
備註：索引有兩個做用：排序和查找

致使額外的文件排序（會下降性能）
explain select name,age from user where name = 'zhangsan' order by pos;//違反最左前綴法則
explain select name,age from user where name = 'zhangsan' order by pos,age;//違反最左前綴法則
explain select * from user where name = 'zhangsan' and age = 20 order by created_time,age;//含非索引字段

十、group by

正常（索引參與了排序）
explain select name,age from user where name = 'zhangsan' group by age;
備註：分組以前必排序（排序同order by）

致使產生臨時表（會下降性能）
explain select name,pos from user where name = 'zhangsan' group by pos;//違反最左前綴法則
explain select name,age from user where name = 'zhangsan' group by pos,age;//違反最左前綴法則
explain select name,age from user where name = 'zhangsan' group by age,created_time;//含非索引字段

使用的示例數據

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