MySql学习笔记（八）：explain之extra

extra主要有是那种情况：Using index、Using filesort、Using temporary、Using where

Using where无需多说，就是使用了where筛选条件。

数据准备：

CREATE TABLE `t_blog` (  `id` int(11) NOT NULL auto_increment,  `title` varchar(50) default NULL,  `typeId` int(11) default NULL,  `a` int(11) default '0',  PRIMARY KEY  (`id`),  KEY `index_1` (`title`,`typeId`)) ENGINE=InnoDB DEFAULT CHARSET=utf8

1、Using index

表示在查询中使用了覆盖索引，避免了扫描表的数据行。

mysql> EXPLAIN select title from t_blog;+----+-------------+--------+-------+---------------+---------+---------+------+------+-------------+| id | select_type | table  | type  | possible_keys | key     | key_len | ref  | rows | Extra       |+----+-------------+--------+-------+---------------+---------+---------+------+------+-------------+|  1 | SIMPLE      | t_blog | index | NULL          | index_1 | 158     | NULL |    7 | Using index |+----+-------------+--------+-------+---------------+---------+---------+------+------+-------------+1 row in set

已知title字段是index_1索引的一部分，上条sql只查询title字段，只会扫描索引文件而不会扫描表的所有数据行，在extra列中，出现了Using index。

mysql> EXPLAIN select * from t_blog;+----+-------------+--------+------+---------------+------+---------+------+------+-------+| id | select_type | table  | type | possible_keys | key  | key_len | ref  | rows | Extra |+----+-------------+--------+------+---------------+------+---------+------+------+-------+|  1 | SIMPLE      | t_blog | ALL  | NULL          | NULL | NULL    | NULL |    7 |       |+----+-------------+--------+------+---------------+------+---------+------+------+-------+1 row in set

上条语句中，除了查询已经加了索引的字段，还查询了没有加索引的字段【a】，导致扫描了表的数据行，因此，extra列中没有出现Using index。

当只出现Using index，没出现Using where时，表示索引用于读取数据，以第一条sql为例。

当Using index 和 Using where同时出现时，表示索引用于查找动作，例如：

mysql> EXPLAIN select title from t_blog where title = 'java';+----+-------------+--------+------+---------------+---------+---------+-------+------+--------------------------+| id | select_type | table  | type | possible_keys | key     | key_len | ref   | rows | Extra                    |+----+-------------+--------+------+---------------+---------+---------+-------+------+--------------------------+|  1 | SIMPLE      | t_blog | ref  | index_1       | index_1 | 153     | const |    1 | Using where; Using index |+----+-------------+--------+------+---------------+---------+---------+-------+------+--------------------------+1 row in set

2、Using filesort

Using filesort通常出现在order by，当试图对一个不是索引的字段进行排序时，mysql就会自动对该字段进行排序，这个过程就称为"文件排序"

mysql> EXPLAIN select * from t_blog order by title;+----+-------------+--------+-------+---------------+---------+---------+------+------+-------+| id | select_type | table  | type  | possible_keys | key     | key_len | ref  | rows | Extra |+----+-------------+--------+-------+---------------+---------+---------+------+------+-------+|  1 | SIMPLE      | t_blog | index | NULL          | index_1 | 158     | NULL |    7 |       |+----+-------------+--------+-------+---------------+---------+---------+------+------+-------+1 row in set

已知title是index_1索引中的第一列索引，所以单独使用时索引生效，在排序时根据索引排序，不会产生文件排序。

mysql> EXPLAIN select * from t_blog order by typeId;+----+-------------+--------+------+---------------+------+---------+------+------+----------------+| id | select_type | table  | type | possible_keys | key  | key_len | ref  | rows | Extra          |+----+-------------+--------+------+---------------+------+---------+------+------+----------------+|  1 | SIMPLE      | t_blog | ALL  | NULL          | NULL | NULL    | NULL |    7 | Using filesort |+----+-------------+--------+------+---------------+------+---------+------+------+----------------+1 row in set

虽然typeId是index_1索引的第二列，但由于缺失第一列，所以索引失效。在排序时无法根据索引排序，故mysql会自动进行排序，产生文件排序。

mysql> EXPLAIN select * from t_blog order by a;+----+-------------+--------+------+---------------+------+---------+------+------+----------------+| id | select_type | table  | type | possible_keys | key  | key_len | ref  | rows | Extra          |+----+-------------+--------+------+---------------+------+---------+------+------+----------------+|  1 | SIMPLE      | t_blog | ALL  | NULL          | NULL | NULL    | NULL |    7 | Using filesort |+----+-------------+--------+------+---------------+------+---------+------+------+----------------+1 row in set

字段a上没有任何索引，所以在排序时无法根据索引排序，因此产生文件排序。

Using filesort出现的情况：排序时无法根据索引进行排序，mysql优化器只能自己进行排序，这种情况会大大降低性能，不可取。

3、Using temporary

表示在查询过程中产生了临时表用于保存中间结果。mysql在对查询结果进行排序时会使用临时表，常见于group by。

group by的实质是先排序后分组，同order by一样，group by和索引息息相关。

试图对一个没有索引的字段进行分组，会产生临时表：

mysql> EXPLAIN select title from t_blog group by typeId;+----+-------------+--------+-------+---------------+---------+---------+------+------+----------------------------------------------+| id | select_type | table  | type  | possible_keys | key     | key_len | ref  | rows | Extra                                        |+----+-------------+--------+-------+---------------+---------+---------+------+------+----------------------------------------------+|  1 | SIMPLE      | t_blog | index | NULL          | index_1 | 158     | NULL |    7 | Using index; Using temporary; Using filesort |+----+-------------+--------+-------+---------------+---------+---------+------+------+----------------------------------------------+1 row in set

对一个有索引的字段进行分组就不会产生临时表：

mysql> EXPLAIN select title from t_blog group by title,typeId;+----+-------------+--------+-------+---------------+---------+---------+------+------+-------------+| id | select_type | table  | type  | possible_keys | key     | key_len | ref  | rows | Extra       |+----+-------------+--------+-------+---------------+---------+---------+------+------+-------------+|  1 | SIMPLE      | t_blog | index | NULL          | index_1 | 158     | NULL |    7 | Using index |+----+-------------+--------+-------+---------------+---------+---------+------+------+-------------+1 row in set

当order by子句和group by子句的字段相同时不会产生临时表：

mysql> explain select * from t_blog b left join t_type t on b.typeId = t.id group by b.id order by b.id;+----+-------------+-------+--------+---------------+---------+---------+---------------+------+-------+| id | select_type | table | type   | possible_keys | key     | key_len | ref           | rows | Extra |+----+-------------+-------+--------+---------------+---------+---------+---------------+------+-------+|  1 | SIMPLE      | b     | index  | NULL          | PRIMARY | 4       | NULL          |    7 |       ||  1 | SIMPLE      | t     | eq_ref | PRIMARY       | PRIMARY | 4       | blog.b.typeId |    1 |       |+----+-------------+-------+--------+---------------+---------+---------+---------------+------+-------+2 rows in set

当order by子句和group by子句的字段不同时就会产生临时表：

mysql> explain select * from t_blog b left join t_type t on b.typeId = t.id group by b.id order by b.title;+----+-------------+-------+--------+---------------+---------+---------+---------------+------+-----------------+| id | select_type | table | type   | possible_keys | key     | key_len | ref           | rows | Extra           |+----+-------------+-------+--------+---------------+---------+---------+---------------+------+-----------------+|  1 | SIMPLE      | b     | index  | NULL          | index_1 | 158     | NULL          |    7 | Using temporary ||  1 | SIMPLE      | t     | eq_ref | PRIMARY       | PRIMARY | 4       | blog.b.typeId |    1 |                 |+----+-------------+-------+--------+---------------+---------+---------+---------------+------+-----------------+2 rows in set

当时用left join时，若order by子句和group by子句都来自于从表时会产生临时表：

mysql> explain select * from t_blog b left join t_type t on b.typeId = t.id group by t.id order by t.id;+----+-------------+-------+--------+---------------+---------+---------+---------------+------+---------------------------------+| id | select_type | table | type   | possible_keys | key     | key_len | ref           | rows | Extra                           |+----+-------------+-------+--------+---------------+---------+---------+---------------+------+---------------------------------+|  1 | SIMPLE      | b     | ALL    | NULL          | NULL    | NULL    | NULL          |    7 | Using temporary; Using filesort ||  1 | SIMPLE      | t     | eq_ref | PRIMARY       | PRIMARY | 4       | blog.b.typeId |    1 |                                 |+----+-------------+-------+--------+---------------+---------+---------+---------------+------+---------------------------------+2 rows in set

mysql> explain select * from t_blog b left join t_type t on b.typeId = t.id group by t.id order by t.name;+----+-------------+-------+--------+---------------+---------+---------+---------------+------+---------------------------------+| id | select_type | table | type   | possible_keys | key     | key_len | ref           | rows | Extra                           |+----+-------------+-------+--------+---------------+---------+---------+---------------+------+---------------------------------+|  1 | SIMPLE      | b     | ALL    | NULL          | NULL    | NULL    | NULL          |    7 | Using temporary; Using filesort ||  1 | SIMPLE      | t     | eq_ref | PRIMARY       | PRIMARY | 4       | blog.b.typeId |    1 |                                 |+----+-------------+-------+--------+---------------+---------+---------+---------------+------+---------------------------------+2 rows in set

出现Using temporary意味着产生了临时表存储中间结果并且最后删掉了该临时表，这个过程很消耗性能。