面試題： MySQL 索引失效的10大原因

本文轉載自查看原文 2020-02-08 15:46 5250 mysql/ 索引失效/ 面試題

個人博客網：https://wushaopei.github.io/ (你想要這里多有)

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 '年齡', pos VARCHAR (20) NOT NULL DEFAULT '' COMMENT '職位', add_time TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP COMMENT '入職時間') CHARSET utf8 COMMENT '員工記錄表' ; )

2、插入數據

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、創建索引

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、全值匹配我最愛

2、最佳左前綴法則

如果索引了多列，要遵守此規則，指的是從索引最左前列開始並且不跳過索引中的列
帶頭大哥不能死，中間兄弟不能斷

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

計算、函數、（自動或手動）類型轉換，會導致索引失效而轉向全表掃描

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、存儲引擎不能使用索引中范圍條件右邊的列

范圍之后全失效

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、盡量使用覆蓋索引（只訪問索引的查詢（索引列和查詢列一直）），減少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在使用不等於（！=或<>）時候，無法使用索引導致全表掃描

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

從 type 列可知，使用了全表掃描，此時沒有使用索引。

7、is null，is not null也無法使用索引

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

由於該列默認不為null，所以默認在使用 is null 時type 為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

當使用 not null 時，會進行全文掃描，查找不是null 的項，此時索引失效。

8、like以通配符開頭，mysql索引失效會編程全表掃描的操作

百分like加右邊

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、字符串不加單引號，索引失效

避免隱式類型轉換

10、少用or，用它連接時索引會失效

免責聲明！

本站轉載的文章為個人學習借鑒使用，本站對版權不負任何法律責任。如果侵犯了您的隱私權益，請聯系本站郵箱yoyou2525@163.com刪除。

猜您在找 Mysql索引面試題 mysql索引面試題 Mysql索引面試題 mysql索引的面試題 mysql索引優化面試題 mysql 索引失效原因面試題整理-MYSQL索引 MySQL索引失效原因【MySQL面試】10道不得不會的MySQL基礎面試題 mysql索引失效的幾種原因