2017-06-02

DB / mysql

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170602-MySQL索引失效情况

MySQL索引失效情况

explain命令解释

mysql> explain select * from u_user where name like '王%' ;
+----+-------------+--------+-------+---------------+----------+---------+------+------+-----------------------+
| id | select_type | table  | type  | possible_keys | key      | key_len | ref  | rows | Extra                 |
+----+-------------+--------+-------+---------------+----------+---------+------+------+-----------------------+
|  1 | SIMPLE      | u_user | range | idx_name      | idx_name | 152     | NULL |   17 | Using index condition |
+----+-------------+--------+-------+---------------+----------+---------+------+------+-----------------------+
1 row in set (0.02 sec)

以下通过explain显示出mysql执行的字段内容：

id: SELECT 查询的标识符. 每个 SELECT 都会自动分配一个唯一的标识符.

select_type: SELECT 查询的类型.

table: 查询的是哪个表

partitions: 匹配的分区

type: join 类型

possible_keys: 此次查询中可能选用的索引

key: 此次查询中确切使用到的索引.

ref: 哪个字段或常数与 key 一起被使用

rows: 显示此查询一共扫描了多少行. 这个是一个估计值.

filtered: 表示此查询条件所过滤的数据的百分比

extra: 额外的信息

索引失效情况

like 以%开头，索引无效；当like前缀没有%，后缀有%时，索引有效

mysql> explain select * from u_user where name like '%王' ;
+----+-------------+--------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table  | type | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+--------+------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | u_user | ALL  | NULL          | NULL | NULL    | NULL | 4613 | Using where |
+----+-------------+--------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)

mysql> explain select * from u_user where name like '王%' ;
+----+-------------+--------+-------+---------------+----------+---------+------+------+-----------------------+
| id | select_type | table  | type  | possible_keys | key      | key_len | ref  | rows | Extra                 |
+----+-------------+--------+-------+---------------+----------+---------+------+------+-----------------------+
|  1 | SIMPLE      | u_user | range | idx_name      | idx_name | 152     | NULL |   17 | Using index condition |
+----+-------------+--------+-------+---------------+----------+---------+------+------+-----------------------+
1 row in set (0.01 sec)

or语句前后没有同时使用索引。当or左右查询字段只有一个是索引，该索引失效，只有当or左右查询字段均为索引时，才会生效

mysql> explain select * from u_user where name ='王五' or user_id='1111'  ;
+----+-------------+--------+-------------+------------------+------------------+---------+------+------+--------------------------------------------+
| id | select_type | table  | type        | possible_keys    | key              | key_len | ref  | rows | Extra                                      |
+----+-------------+--------+-------------+------------------+------------------+---------+------+------+--------------------------------------------+
|  1 | SIMPLE      | u_user | index_merge | user_id,idx_name | idx_name,user_id | 152,92  | NULL |    2 | Using union(idx_name,user_id); Using where |
+----+-------------+--------+-------------+------------------+------------------+---------+------+------+--------------------------------------------+
1 row in set (0.08 sec)

mysql> explain select * from u_user where name ='王五' or level_cd='11'  ;
+----+-------------+--------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table  | type | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+--------+------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | u_user | ALL  | idx_name      | NULL | NULL    | NULL | 4613 | Using where |
+----+-------------+--------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.01 sec)

组合索引，不是使用第一列索引，索引失效

mysql> explain select * from u_user where email ='yubo@163.com';
+----+-------------+--------+------+-------------------+-------------------+---------+-------+------+-----------------------+
| id | select_type | table  | type | possible_keys     | key               | key_len | ref   | rows | Extra                 |
+----+-------------+--------+------+-------------------+-------------------+---------+-------+------+-----------------------+
|  1 | SIMPLE      | u_user | ref  | idx_email_mobile | idx_email_mobile | 93      | const |    1 | Using index condition |
+----+-------------+--------+------+-------------------+-------------------+---------+-------+------+-----------------------+
1 row in set (0.00 sec)

mysql> explain select * from u_user where tel ='17797173884';
+----+-------------+--------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table  | type | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+--------+------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | u_user | ALL  | NULL          | NULL | NULL    | NULL | 4613 | Using where |
+----+-------------+--------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.02 sec)

数据类型出现隐式转化。如varchar不加单引号的话可能会自动转换为int型，使索引无效，产生全表扫描

mysql> explain select * from u_user where name='17';
+----+-------------+--------+------+---------------+----------+---------+-------+------+-----------------------+
| id | select_type | table  | type | possible_keys | key      | key_len | ref   | rows | Extra                 |
+----+-------------+--------+------+---------------+----------+---------+-------+------+-----------------------+
|  1 | SIMPLE      | u_user | ref  | idx_name      | idx_name | 152     | const |    1 | Using index condition |
+----+-------------+--------+------+---------------+----------+---------+-------+------+-----------------------+
1 row in set (0.01 sec)

mysql> explain select * from u_user where name=17;
+----+-------------+--------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table  | type | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+--------+------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | u_user | ALL  | idx_name      | NULL | NULL    | NULL | 4613 | Using where |
+----+-------------+--------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)

在索引列上使用 IS NULL 或 IS NOT NULL操作。索引是不索引空值的，所以这样的操作不能使用索引，可以用其他的办法处理，例如：数字类型，判断大于0，字符串类型设置一个默认值，判断是否等于默认值即可。

mysql> explain select * from u_user where email is  null;
+----+-------------+--------+------+---------------+-----------+---------+-------+------+-----------------------+
| id | select_type | table  | type | possible_keys | key       | key_len | ref   | rows | Extra                 |
+----+-------------+--------+------+---------------+-----------+---------+-------+------+-----------------------+
|  1 | SIMPLE      | u_user | ref  | idx_email     | idx_email | 93      | const |  116 | Using index condition |
+----+-------------+--------+------+---------------+-----------+---------+-------+------+-----------------------+
1 row in set (0.01 sec)

mysql> explain select * from u_user where email is not null;
+----+-------------+--------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table  | type | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+--------+------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | u_user | ALL  | idx_email     | NULL | NULL    | NULL | 4613 | Using where |
+----+-------------+--------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.03 sec)

在索引字段上使用not，<>，!=。不等于操作符是永远不会用到索引的，因此对它的处理只会产生全表扫描。优化方法： key<>0 改为 key>0 or key<0

mysql> explain select * from u_user where age !=20;
+----+-------------+--------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table  | type | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+--------+------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | u_user | ALL  | idx_age       | NULL | NULL    | NULL | 4613 | Using where |
+----+-------------+--------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.09 sec)

mysql> explain select * from u_user where age <20;
+----+-------------+--------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table  | type | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+--------+------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | u_user | ALL  | idx_age       | NULL | NULL    | NULL | 4613 | Using where |
+----+-------------+--------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.06 sec)

mysql> explain select * from u_user where age >20;
+----+-------------+--------+-------+---------------+---------+---------+------+------+-----------------------+
| id | select_type | table  | type  | possible_keys | key     | key_len | ref  | rows | Extra                 |
+----+-------------+--------+-------+---------------+---------+---------+------+------+-----------------------+
|  1 | SIMPLE      | u_user | range | idx_age       | idx_age | 5       | NULL |    4 | Using index condition |
+----+-------------+--------+-------+---------------+---------+---------+------+------+-----------------------+
1 row in set (0.02 sec)

对索引字段进行计算操作、字段上使用函数

mysql> explain select name from u_user  where left(email,1) ='a';
+----+-------------+--------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table  | type | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+--------+------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | u_user | ALL  | NULL          | NULL | NULL    | NULL | 4613 | Using where |
+----+-------------+--------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.03 sec)

当全表扫描速度比索引速度快时，mysql会使用全表扫描，此时索引失效

索引失效分析工具

索引失效分析工具：

可以使用explain命令加在要分析的sql语句前面，在执行结果中查看key这一列的值，如果为NULL，说明没有使用索引。

参考资料

Mysql索引查询失效的情况

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