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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

查看全文

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原文地址：https://www.cnblogs.com/wushaopei/p/12283647.html