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
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