mysql視頻教程欄目索引失效的情況。
索引對于mysql而言，是非常重要的篇章。索引知識點也巨多，要想掌握透徹，需要逐個知識點一一擊破，今天來先來聊聊哪些情況下會導致索引失效。圖片總結版
相關免費學習推薦：mysql視頻教程
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';
1、違反最左前綴法則
如果索引有多列，要遵守最左前綴法則 即查詢從索引的最左前列開始并且不跳過索引中的列 explain select * from user where age = 20 and phone = '18730658760' and pos = 'cxy';
2、在索引列上做任何操作
如計算、函數、（自動or手動）類型轉換等操作，會導致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';
3、索引范圍條件右邊的列
索引范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';
4、盡量使用覆蓋索引
只訪問索引查詢（索引列和查詢列一致），減少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（!=、<>）的時候無法使用索引會導致全表掃描（除覆蓋索引外） explain select * from user where age != 20; explain select * from user where age <> 20;
6、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%';
7、字符串不加單引號索引失效
explain select * from user where name = 2000;
8、or連接
少用or explain select * from user where name = '2000' or age = 20 or pos ='cxy';
9、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; 備注：索引有兩個作用：排序和查找
導致額外的文件排序（會降低性能） 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
10、group by
正常（索引參與了排序） explain select name,age from user where name = 'zhangsan' group by age; 備注：分組之前必排序（排序同order by）
導致產生臨時表（會降低性能） 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
? 版權聲明
文章版權歸作者所有，未經允許請勿轉載。
THE END
喜歡就支持一下吧
相關推薦
