面试题： MySQL 索引失效的10大原因
个人博客网：https://wushaopei.github.io/ (你想要这里多有)
1、建表：
CREATE TABLE staffs (
id INT PRIMARY KEY AUTO_INCREMENT,
NAME VARCHAR (24) NOT NULL DEFAULT '' COMMENT '姓名',
age INT NOT NULL DEFAULT 0 COMMENT '年龄',
pos VARCHAR (20) NOT NULL DEFAULT '' COMMENT '职位',
add_time TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP COMMENT '入职时间') CHARSET utf8 COMMENT '员工记录表' ;
)
2、插入数据
INSERT INTO staffs(NAME,age,pos,add_time) VALUES('z3',22,'manager',NOW());
INSERT INTO staffs(NAME,age,pos,add_time) VALUES('July',23,'dev',NOW());
3、创建索引
ALTER TABLE staffs ADD INDEX idx_staffs_nameAgePos(NAME, age, pos);
基本表信息：
mysql> desc staffs;
+----------+-------------+------+-----+-------------------+-------------------+
| Field | Type | Null | Key | Default | Extra |
+----------+-------------+------+-----+-------------------+-------------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| NAME | varchar(24) | NO | MUL | | |
| age | int(11) | NO | | 0 | |
| pos | varchar(20) | NO | | | |
| add_time | timestamp | NO | | CURRENT_TIMESTAMP | DEFAULT_GENERATED |
+----------+-------------+------+-----+-------------------+-------------------+
5 rows in set
mysql> select * from staffs;
+----+------+-----+---------+---------------------+
| id | NAME | age | pos | add_time |
+----+------+-----+---------+---------------------+
| 1 | z3 | 22 | manager | 2020-01-22 21:46:31 |
| 2 | July | 23 | dev | 2020-01-22 21:46:31 |
| 3 | h4 | 22 | manager | 2020-01-22 21:57:38 |
| 4 | Juue | 43 | dev | 2020-01-22 21:57:38 |
| ................. |
+----+------+-----+---------+---------------------+
198 rows in set
show index from staffs;
+--------+------------+-----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | Visible | Expression |
+--------+------------+-----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
| staffs | 0 | PRIMARY | 1 | id | A | 2 | NULL | NULL | | BTREE | | | YES | NULL |
| staffs | 1 | idx_staffs_nameAgePos | 1 | NAME | A | 2 | NULL | NULL | | BTREE | | | YES | NULL |
| staffs | 1 | idx_staffs_nameAgePos | 2 | age | A | 2 | NULL | NULL | | BTREE | | | YES | NULL |
| staffs | 1 | idx_staffs_nameAgePos | 3 | pos | A | 2 | NULL | NULL | | BTREE | | | YES | NULL |
+--------+------------+-----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
4 rows in set
1、全值匹配我最爱
2、最佳左前缀法则
- 如果索引了多列，要遵守此规则，指的是从索引最左前列开始并且不跳过索引中的列
- 带头大哥不能死，中间兄弟不能断
mysql> explain select * from staffs where age=28 and pos='managerment';
+----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------------+
| 1 | SIMPLE | staffs | NULL | ALL | NULL | NULL | NULL | NULL | 198 | 1 | Using where |
+----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set , 1 warning (0.001 sec)
3、不在索引列上做任何操作
- 计算、函数、（自动或手动）类型转换，会导致索引失效而转向全表扫描
mysql> explain select * from staffs where name= 'hu';
+----+-------------+--------+------------+------+-----------------------+-----------------------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+--------+------------+------+-----------------------+-----------------------+---------+-------+------+----------+-------+
| 1 | SIMPLE | staffs | NULL | ref | idx_staffs_nameAgePos | idx_staffs_nameAgePos | 74 | const | 1 | 100 | NULL |
+----+-------------+--------+------------+------+-----------------------+-----------------------+---------+-------+------+----------+-------+
1 row in set , 1 warning (0.00 sec)
mysql> explain select * from st
affs where left(name,4)= 'hu';
+----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------------+
| 1 | SIMPLE | staffs | NULL | ALL | NULL | NULL | NULL | NULL | 198 | 100 | Using where |
+----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set , 1 warning (0.00 sec)
4、存储引擎不能使用索引中范围条件右边的列
- 范围之后全失效
mysql> explain select * from staffs where name
='hu' and age=28;
+----+-------------+--------+------------+------+-----------------------+-----------------------+---------+-------------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+--------+------------+------+-----------------------+-----------------------+---------+-------------+------+----------+-------+
| 1 | SIMPLE | staffs | NULL | ref | idx_staffs_nameAgePos | idx_staffs_nameAgePos | 78 | const,const | 1 | 100 | NULL |
+----+-------------+--------+------------+------+-----------------------+-----------------------+---------+-------------+------+----------+-------+
1 row in set
mysql> explain select * from staffs where name='hu' and
age=28 and pos='managerment';
+----+-------------+--------+------------+------+-----------------------+-----------------------+---------+-------------------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+--------+------------+------+-----------------------+-----------------------+---------+-------------------+------+----------+-------+
| 1 | SIMPLE | staffs | NULL | ref | idx_staffs_nameAgePos | idx_staffs_nameAgePos | 140 | const,const,const | 1 | 100 | NULL |
+----+-------------+--------+------------+------+-----------------------+-----------------------+---------+-------------------+------+----------+-------+
1 row in set
5、尽量使用覆盖索引（只访问索引的查询（索引列和查询列一直）），减少select *
mysql> explain select * from staffs where name='hu' and
age=28 and pos='managerment';
+----+-------------+--------+------------+------+-----------------------+-----------------------+---------+-------------------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+--------+------------+------+-----------------------+-----------------------+---------+-------------------+------+----------+-------+
| 1 | SIMPLE | staffs | NULL | ref | idx_staffs_nameAgePos | idx_staffs_nameAgePos | 140 | const,const,const | 1 | 100 | NULL |
+----+-------------+--------+------------+------+-----------------------+-----------------------+---------+-------------------+------+----------+-------+
1 row in set
mysql> explain select name,age,pos
from staffs where name='hu' and age=28 and pos='managerment';
+----+-------------+--------+------------+------+-----------------------+-----------------------+---------+-------------------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+--------+------------+------+-----------------------+-----------------------+---------+-------------------+------+----------+-------------+
| 1 | SIMPLE | staffs | NULL | ref | idx_staffs_nameAgePos | idx_staffs_nameAgePos | 140 | const,const,const | 1 | 100 | Using index |
+----+-------------+--------+------------+------+-----------------------+-----------------------+---------+-------------------+------+----------+-------------+
1 row in set
6、mysql在使用不等于（！=或<>）时候，无法使用索引导致全表扫描
mysql> explain select * from staffs where name='hu'
;
+----+-------------+--------+------------+------+-----------------------+-----------------------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+--------+------------+------+-----------------------+-----------------------+---------+-------+------+----------+-------+
| 1 | SIMPLE | staffs | NULL | ref | idx_staffs_nameAgePos | idx_staffs_nameAgePos | 74 | const | 1 | 100 | NULL |
+----+-------------+--------+------------+------+-----------------------+-----------------------+---------+-------+------+----------+-------+
1 row in set
mysql> explain select * from staffs where name!='hu' ;
+----+-------------+--------+------------+------+-----------------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+--------+------------+------+-----------------------+------+---------+------+------+----------+-------------+
| 1 | SIMPLE | staffs | NULL | ALL | idx_staffs_nameAgePos | NULL | NULL | NULL | 198 | 99.49 | Using where |
+----+-------------+--------+------------+------+-----------------------+------+---------+------+------+----------+-------------+
1 row in set
mysql> explain select * from staffs where name<>'hu' ;
+----+-------------+--------+------------+------+-----------------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+--------+------------+------+-----------------------+------+---------+------+------+----------+-------------+
| 1 | SIMPLE | staffs | NULL | ALL | idx_staffs_nameAgePos | NULL | NULL | NULL | 198 | 99.49 | Using where |
+----+-------------+--------+------------+------+-----------------------+------+---------+------+------+----------+-------------+
1 row in set
从 type 列可知，使用了全表扫描，此时没有使用索引。
7、is null，is not null也无法使用索引
mysql> explain select * from staffs where name is null ;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------+
| 1 | SIMPLE | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Impossible WHERE |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------+
1 row in set
由于该列默认不为null，所以默认在使用 is null 时type 为null
mysql> explain select * from staffs where name is not null ;
+----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------+
| 1 | SIMPLE | staffs | NULL | ALL | NULL | NULL | NULL | NULL | 198 | 100 | NULL |
+----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------+
1 row in set
当使用 not null 时，会进行全文扫描，查找不是null 的项，此时索引失效。
8、like以通配符开头，mysql索引失效会编程全表扫描的操作
- 百分like加右边
mysql> explain select * from staffs where name like '%hu%' ;
+----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------------+
| 1 | SIMPLE | staffs | NULL | ALL | NULL | NULL | NULL | NULL | 198 | 11.11 | Using where |
+----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set
mysql> explain select * from staffs where name like '%hu' ;
+----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------------+
| 1 | SIMPLE | staffs | NULL | ALL | NULL | NULL | NULL | NULL | 198 | 11.11 | Using where |
+----+-------------+--------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set
mysql> explain select * from staffs where name like 'hu%' ;
+----+-------------+--------+------------+-------+-----------------------+-----------------------+---------+------+------+----------+-----------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+--------+------------+-------+-----------------------+-----------------------+---------+------+------+----------+-----------------------+
| 1 | SIMPLE | staffs | NULL | range | idx_staffs_nameAgePos | idx_staffs_nameAgePos | 74 | NULL | 1 | 100 | Using index condition |
+----+-------------+--------+------------+-------+-----------------------+-----------------------+---------+------+------+----------+-----------------------+
1 row in set
9、字符串不加单引号，索引失效
- 避免隐式类型转换
10、少用or，用它连接时索引会失效
面试题： MySQL 索引失效的10大原因的更多相关文章
- mysql索引总结(4)-MySQL索引失效的几种情况
mysql索引总结(1)-mysql 索引类型以及创建 mysql索引总结(2)-MySQL聚簇索引和非聚簇索引 mysql索引总结(3)-MySQL聚簇索引和非聚簇索引 mysql索引总结(4)-M ...
- MySQL索引失效的常见场景
当然请记住,explain是一个好习惯! MySQL索引失效的常见场景 在验证下面的场景时,请准备足够多的数据量,因为数据量少时,MySQL的优化器有时会判定全表扫描无伤大雅,就不会命中索引了. 1. ...
- 面试突击60：什么情况会导致 MySQL 索引失效？
为了验证 MySQL 中哪些情况下会导致索引失效,我们可以借助 explain 执行计划来分析索引失效的具体场景. explain 使用如下,只需要在查询的 SQL 前面添加上 explain 关键字 ...
- MySQL索引失效的几种情况
1.索引不存储null值 更准确的说,单列索引不存储null值,复合索引不存储全为null的值.索引不能存储Null,所以对这列采用is null条件时,因为索引上根本 没Null值,不能利用到索引, ...
- 【索引失效】什么情况下会引起MySQL索引失效
索引并不是时时都会生效的,比如以下几种情况,将导致索引失效: 1.如果条件中有or,即使其中有条件带索引也不会使用(这也是为什么尽量少用or的原因) 注意:要想使用or,又想让索引生效,只能将or条件 ...
- 【转-mysql索引失效的几种情形】
索引并不是时时都会生效的,比如以下几种情况,将导致索引失效: 1.如果条件中有or,即使其中有条件带索引也不会使用(这也是为什么尽量少用or的原因) 注意:要想使用or,又想让索引生效,只能将or条件 ...
- 3、MySQL 索引失效的场景
索引失效的场景: 1.没有 where 条件 直接看 SQL 语句 2.where 条件中所在的列没有建立索引 show index from t; 3.从表中取得数据超过某个阈值.通常认为是 ...
- MySQL索引失效之隐式转换
常见索引失效: 1. 条件索引字段"不干净":函数操作.运算操作 2. 隐式类型转换:字符串转数值:其他类型转换 3. 隐式字符编码转换:按字符编码数据长度大的方向转换,避免数据截 ...
- MySQL 索引失效-模糊查询，最左匹配原则，OR条件等。
索引失效 介绍 索引失效就是我们明明在查询时的条件为索引列(包括自己新建的索引),但是索引不能起效,走的是全表扫描.explain 后可查看type=ALL. 这是为什么呢? 首先介绍有以下几种情况索 ...
随机推荐
- leetcode240——搜索二维矩阵（medium)
一.题目描述 编写一个高效的算法来搜索 m x n 矩阵 matrix 中的一个目标值 target.该矩阵具有以下特性: 每行的元素从左到右升序排列. 每列的元素从上到下升序排列. 示例: 现有矩阵 ...
- C# 数据操作系列 - 1. SQL基础操作
0.前言 前篇介绍了一些数据库的基本概念和以及一些常见的数据库,让我们对数据库有了一个初步的认识.这一篇我们将继续为C#数据操作的基础填上一个空白-SQL语句. SQL(Structured Quer ...
- 【Spark】一张图看懂Spark的运行架构，以standAlone模式为例
- Day_11【集合】扩展案例2_使用普通for循环获取集合中索引为3的元素并打印，统计集合中包含字符串"def"的数量，删除集合中的所有字符串"，将集合中每个元素中的小写字母变成大写字母def"，
分析以下需求,并用代码实现 1.定义ArrayList集合,存入多个字符串"abc" "def" "efg" "def" ...
- js console一些常用的功能
前言 很多时候我们在调试的时候经常用console.log,我感觉其实一个就够了,但是有时候你不可能写一步就去调试下,所以呢,经常用几个console.log,有时候挺难找的,后面翻了翻console ...
- python --字符串学习
一 转义字符 借用一个特殊的方法表示一系列不方便写出的内容,比如回车键,换行符,退格键 借助反斜杠字符,一旦出现反斜杠,则反斜杠后面一个或者几个字符表示已经不是原来的意思了 在字符串中,一旦出现了斜杠 ...
- pc建站自适应
转载 来自https://www.cnblogs.com/eyed/p/7872521.html HTML5----响应式(自适应)网页设计 现在,很多项目都需要做响应式或者自适应的来适应 ...
- Js调用Android回调处理
通常在混合app中经常会使用js调用native的方法,一般是: window.nativeApp.call(XXX); 直接调用native方法,对于简单的处理倒是可以,如果需要回调呢?期待的方式是 ...
- Mybatis 注入全局参数
在项目中使用mybatis作为dao层,大部分时间都需要使用到mybatis提供的动态sql功能,一般情况下所有的表都是在同一个数据库下的,进行数据操作时都是使用jdbc中默认的schema.但是如果 ...
- vue element-ui el-form-item 循环渲染，验证表单内容
data里面如下图: