MySQL执行计划解析（四）

本文是对于MySQL执行计划的解析，主要解释了MySQL执行计划中的各个参数及含义。

十三、Extra

产生的值

存在六种情况：

Using filesort、Using temporary、use index、using where、using join buffer、impossible where

1、Using filesort

说明mysql会对数据使用一个外部的索引排序,而不是按照表内的索引顺序进行，

Mysql中无法利用索引完成排序操作称为"文件排序"。

EXPLAIN SELECT * FROM EMPLOYEE ORDER BY SALARY;+----+-------------+----------+------------+------+---------------+------+---------+------+------+----------+----------------+| id | select_type | table    | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra          |+----+-------------+----------+------------+------+---------------+------+---------+------+------+----------+----------------+|  1 | SIMPLE      | EMPLOYEE | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    8 |   100.00 | Using filesort |+----+-------------+----------+------------+------+---------------+------+---------+------+------+----------+----------------+1 row in set, 1 warning (0.02 sec)

根据salary字段进行排序，并且salary字段上没有索引，那么此时会使用文件排序，extra为using filesort。

2、Using temporary

使用了临时表保存中间结果,Mysql在对查询结果排序时, 使用了临时表,常见于排序orderby 和分组查询group by。

查看dep_id的数据分布：

EXPLAIN SELECT DEP_ID      ,COUNT(*)FROM   EMPLOYEEGROUP  BY DEP_ID;+----+-------------+----------+------------+------+---------------+------+---------+------+------+----------+-----------------+| id | select_type | table    | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra           |+----+-------------+----------+------------+------+---------------+------+---------+------+------+----------+-----------------+|  1 | SIMPLE      | EMPLOYEE | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    8 |   100.00 | Using temporary |+----+-------------+----------+------------+------+---------------+------+---------+------+------+----------+-----------------+1 row in set, 1 warning (0.01 sec)

对dep_id进行了group by，此时需要对中间的结果进行保存，所以会使用临时表，extra为using temporary。

3、use index

表示相应的select中使用了覆盖索引,避免访问了表的数据行, 效率很好。

如果同时出现using where，表明索引被用来执行索引键值的查找；

如果没有同时出现using where，表明索引用来读取数据而非执行查找动作。

对工资升序查找数据（有索引）：

EXPLAIN SELECT SALARY FROM EMPLOYEE ORDER BY SALARY;+----+-------------+----------+------------+------+---------------+------+---------+------+------+----------+----------------+| id | select_type | table    | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra          |+----+-------------+----------+------------+------+---------------+------+---------+------+------+----------+----------------+|  1 | SIMPLE      | EMPLOYEE | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    8 |   100.00 | Using filesort |+----+-------------+----------+------------+------+---------------+------+---------+------+------+----------+----------------+1 row in set, 1 warning (0.00 sec)

使用salary进行排序，并且有索引，那么通过该列的索引即可查出数据，索引只是用来读取数据的，也避免了排序，

此时extra显示为using index。

4、using where

表明使用了where过滤（过滤字段没有索引或者不能使用索引）。

Dep_id字段没有索引。

EXPLAIN SELECT * FROM EMPLOYEE WHERE DEP_ID = 4;+----+-------------+----------+------------+------+---------------+------+---------+------+------+----------+-------------+| id | select_type | table    | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |+----+-------------+----------+------------+------+---------------+------+---------+------+------+----------+-------------+|  1 | SIMPLE      | EMPLOYEE | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    8 |    12.50 | Using where |+----+-------------+----------+------------+------+---------------+------+---------+------+------+----------+-------------+1 row in set, 1 warning (0.00 sec)

当有使用where进行过滤时，在extra里使用using where表示。

dep_id创建索引：

CREATE INDEX idx_emp_02 ON employee ( dep_id );当可用正常使用索引时：EXPLAIN SELECT * FROM EMPLOYEE WHERE DEP_ID > 4;+----+-------------+----------+------------+-------+---------------+------------+---------+------+------+----------+-----------------------+| id | select_type | table    | partitions | type  | possible_keys | key        | key_len | ref  | rows | filtered | Extra                 |+----+-------------+----------+------------+-------+---------------+------------+---------+------+------+----------+-----------------------+|  1 | SIMPLE      | EMPLOYEE | NULL       | range | idx_emp_02    | idx_emp_02 | 5       | NULL |    5 |   100.00 | Using index condition |+----+-------------+----------+------------+-------+---------------+------------+---------+------+------+----------+-----------------------+1 row in set, 1 warning (0.00 sec)

通过索引进行访问，标记为Using index condition。

不可以使用索引时：

EXPLAIN SELECT * FROM EMPLOYEE WHERE DEP_ID + 1 > 4;+----+-------------+----------+------------+------+---------------+------+---------+------+------+----------+-------------+| id | select_type | table    | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |+----+-------------+----------+------------+------+---------------+------+---------+------+------+----------+-------------+|  1 | SIMPLE      | EMPLOYEE | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    8 |   100.00 | Using where |+----+-------------+----------+------------+------+---------------+------+---------+------+------+----------+-------------+1 row in set, 1 warning (0.00 sec)

标记为Using where

5、Using index condition

在使用where条件的索引时，会标记为Using index condition。

EXPLAIN SELECT * FROM EMPLOYEE WHERE DEP_ID > 4;+----+-------------+----------+------------+-------+---------------+------------+---------+------+------+----------+-----------------------+| id | select_type | table    | partitions | type  | possible_keys | key        | key_len | ref  | rows | filtered | Extra                 |+----+-------------+----------+------------+-------+---------------+------------+---------+------+------+----------+-----------------------+|  1 | SIMPLE      | EMPLOYEE | NULL       | range | idx_emp_02    | idx_emp_02 | 5       | NULL |    5 |   100.00 | Using index condition |+----+-------------+----------+------------+-------+---------------+------------+---------+------+------+----------+-----------------------+1 row in set, 1 warning (0.00 sec)

通过索引进行访问，标记为Using index condition。

6、impossible where

where条件导致没有返回的行。

EXPLAIN SELECT * FROM EMPLOYEE WHERE ID 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, 1 warning (0.00 sec)

Id是主键，所以不会有空值，在进行一个id为空的查询时，是没有结果的，因此extra会表示为impossible where。