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explain语法

有两种用法：

1.EXPLAIN tbl_name

2.EXPLAIN [EXTENDED] SELECT select_options 为了更好的说明它，我们需要建两张表，下面的语句用于创建一张测试用的订单表：

CREATE TABLE `t_order` (  `order_id` int(10) unsigned NOT NULL AUTO_INCREMENT COMMENT '订单ID',  `express_type` tinyint(1) unsigned NOT NULL COMMENT '快递方式',  `user_id` int(10) unsigned DEFAULT NULL COMMENT '用户ID',  `add_time` int(10) NOT NULL COMMENT '下单时间',  PRIMARY KEY (`order_id`),  KEY `user_id` (`user_id`),  KEY `express_type` (`express_type`) ) ENGINE=InnoDB AUTO_INCREMENT=100000 DEFAULT CHARSET=utf8 COMMENT='订单记录表'

然后我往其中插入了100000条记录。

下面是订单的扩展表，仅向其中插入了一条记录，关联某笔订单。

CREATE TABLE `t_order_ext` (  `order_id` int(10) NOT NULL COMMENT '订单ID',  `user_type` int(11) NOT NULL DEFAULT '0' COMMENT '用户类型',  `comment` varchar(255) CHARACTER SET utf8 NOT NULL COMMENT '订单备注',  KEY `order_id` (`order_id`) ) ENGINE=InnoDB DEFAULT CHARSET=utf8

先看一下第一种用法：

mysql> explain t_order; +--------------+---------------------+------+-----+---------+----------------+ | Field        | Type                | Null | Key | Default | Extra          | +--------------+---------------------+------+-----+---------+----------------+ | order_id     | int(10) unsigned    | NO   | PRI | NULL    | auto_increment | | express_type | tinyint(1) unsigned | NO   | MUL | NULL    |                | | user_id      | int(10) unsigned    | YES  | MUL | NULL    |                | | add_time     | int(10)             | NO   |     | NULL    |                | +--------------+---------------------+------+-----+---------+----------------+ 4 rows in set (0.00 sec)

这其实和describe是等价的:

mysql> describe t_order; +--------------+---------------------+------+-----+---------+----------------+ | Field        | Type                | Null | Key | Default | Extra          | +--------------+---------------------+------+-----+---------+----------------+ | order_id     | int(10) unsigned    | NO   | PRI | NULL    | auto_increment | | express_type | tinyint(1) unsigned | NO   | MUL | NULL    |                | | user_id      | int(10) unsigned    | YES  | MUL | NULL    |                | | add_time     | int(10)             | NO   |     | NULL    |                | +--------------+---------------------+------+-----+---------+----------------+ 4 rows in set (0.00 sec)

这返回的是表的结构，没有什么可说的。

重点是第二种用法，需要深入的了解。

先看一个例子：

mysql> explain select * from t_order; +----+-------------+---------+------+---------------+------+---------+------+--------+-------+ | id | select_type | table   | type | possible_keys | key  | key_len | ref  | rows   | Extra | +----+-------------+---------+------+---------------+------+---------+------+--------+-------+ |  1 | SIMPLE      | t_order | ALL  | NULL          | NULL | NULL    | NULL | 100453 |       | +----+-------------+---------+------+---------------+------+---------+------+--------+-------+ 1 row in set (0.03 sec)

加上extended后之后：

mysql> explain extended select * from t_order; +----+-------------+---------+------+---------------+------+---------+------+--------+----------+-------+ | id | select_type | table   | type | possible_keys | key  | key_len | ref  | rows   | filtered | Extra | +----+-------------+---------+------+---------------+------+---------+------+--------+----------+-------+ |  1 | SIMPLE      | t_order | ALL  | NULL          | NULL | NULL    | NULL | 100453 |   100.00 |       | +----+-------------+---------+------+---------------+------+---------+------+--------+----------+-------+ 1 row in set, 1 warning (0.00 sec)

有必要解释一下这个长长的表格里每一列的含义：

下面对几个重要的参数进行一下说明

一.select_type的说明

1.UNION:

当通过union来连接多个查询结果时，第二个之后的select其select_type为UNION。

mysql> explain select * from t_order where order_id=100 union select * from t_order where order_id=200; +----+--------------+------------+-------+---------------+---------+---------+-------+------+-------+ | id | select_type  | table      | type  | possible_keys | key     | key_len | ref   | rows | Extra | +----+--------------+------------+-------+---------------+---------+---------+-------+------+-------+ |  1 | PRIMARY      | t_order    | const | PRIMARY       | PRIMARY | 4       | const |    1 |       | |  2 | UNION        | t_order    | const | PRIMARY       | PRIMARY | 4       | const |    1 |       | | NULL | UNION RESULT | 
   
     | ALL   | NULL          | NULL    | NULL    | NULL  | NULL |       | +----+--------------+------------+-------+---------------+---------+---------+-------+------+-------+ 3 rows in set (0.34 sec)
   

2.DEPENDENT UNION与DEPENDENT SUBQUERY:

当union作为子查询时，其中第二个union的select_type就是DEPENDENT UNION。

第一个子查询的select_type则是DEPENDENT SUBQUERY。

mysql> explain select * from t_order where order_id in (select order_id from t_order where order_id=100 union select order_id from t_order where order_id=200); +----+--------------------+------------+-------+---------------+---------+---------+-------+--------+-------------+ | id | select_type        | table      | type  | possible_keys | key     | key_len | ref   | rows   | Extra       | +----+--------------------+------------+-------+---------------+---------+---------+-------+--------+-------------+ |  1 | PRIMARY            | t_order    | ALL   | NULL          | NULL    | NULL    | NULL  | 100453 | Using where | |  2 | DEPENDENT SUBQUERY | t_order    | const | PRIMARY       | PRIMARY | 4       | const |      1 | Using index | |  3 | DEPENDENT UNION    | t_order    | const | PRIMARY       | PRIMARY | 4       | const |      1 | Using index | | NULL | UNION RESULT       | 
   
     | ALL   | NULL          | NULL    | NULL    | NULL  |   NULL |             | +----+--------------------+------------+-------+---------------+---------+---------+-------+--------+-------------+ 4 rows in set (0.03 sec)
   

3.SUBQUERY:

子查询中的第一个select其select_type为SUBQUERY。

mysql> explain select * from t_order where order_id=(select order_id from t_order where order_id=100); +----+-------------+---------+-------+---------------+---------+---------+-------+------+-------------+ | id | select_type | table   | type  | possible_keys | key     | key_len | ref   | rows | Extra       | +----+-------------+---------+-------+---------------+---------+---------+-------+------+-------------+ |  1 | PRIMARY     | t_order | const | PRIMARY       | PRIMARY | 4       | const |    1 |             | |  2 | SUBQUERY    | t_order | const | PRIMARY       | PRIMARY | 4       |       |    1 | Using index | +----+-------------+---------+-------+---------------+---------+---------+-------+------+-------------+ 2 rows in set (0.03 sec)

4.DERIVED:

当子查询是from子句时，其select_type为DERIVED。

mysql> explain select * from (select order_id from t_order where order_id=100) a; +----+-------------+------------+--------+---------------+---------+---------+------+------+-------------+ | id | select_type | table      | type   | possible_keys | key     | key_len | ref  | rows | Extra       | +----+-------------+------------+--------+---------------+---------+---------+------+------+-------------+ |  1 | PRIMARY     | 
   
     | system | NULL          | NULL    | NULL    | NULL |    1 |             | |  2 | DERIVED     | t_order    | const  | PRIMARY       | PRIMARY | 4       |      |    1 | Using index | +----+-------------+------------+--------+---------------+---------+---------+------+------+-------------+ 2 rows in set (0.03 sec)
   

二.type的说明

1.system，const

见上面4.DERIVED的例子。其中第一行的type就是为system，第二行是const，这两种联接类型是最快的。

2.eq_ref

在t_order表中的order_id是主键，t_order_ext表中的order_id也是主键，该表可以认为是订单表的补充信息表，他们的关系是1对1，在下面的例子中可以看到b表的连接类型是eq_ref，这是极快的联接类型。

mysql> explain select * from t_order a,t_order_ext b where a.order_id=b.order_id; +----+-------------+-------+--------+---------------+---------+---------+-----------------+------+-------------+ | id | select_type | table | type   | possible_keys | key     | key_len | ref             | rows | Extra       | +----+-------------+-------+--------+---------------+---------+---------+-----------------+------+-------------+ |  1 | SIMPLE      | b     | ALL    | order_id      | NULL    | NULL    | NULL            |    1 |             | |  1 | SIMPLE      | a     | eq_ref | PRIMARY       | PRIMARY | 4       | test.b.order_id |    1 | Using where | +----+-------------+-------+--------+---------------+---------+---------+-----------------+------+-------------+ 2 rows in set (0.00 sec)

3.ref

下面的例子在上面的例子上略作了修改，加上了条件。此时b表的联接类型变成了ref。因为所有与a表中order_id=100的匹配记录都将会从b表获取。这是比较常见的联接类型。

mysql> explain select * from t_order a,t_order_ext b where a.order_id=b.order_id and a.order_id=100; +----+-------------+-------+-------+---------------+----------+---------+-------+------+-------+ | id | select_type | table | type  | possible_keys | key      | key_len | ref   | rows | Extra | +----+-------------+-------+-------+---------------+----------+---------+-------+------+-------+ |  1 | SIMPLE      | a     | const | PRIMARY       | PRIMARY  | 4       | const |    1 |       | |  1 | SIMPLE      | b     | ref   | order_id      | order_id | 4       | const |    1 |       | +----+-------------+-------+-------+---------------+----------+---------+-------+------+-------+ 2 rows in set (0.00 sec)

4.ref_or_null

user_id字段是一个可以为空的字段，并对该字段创建了一个索引。在下面的查询中可以看到联接类型为ref_or_null，这是mysql为含有null的字段专门做的处理。在我们的表设计中应当尽量避免索引字段为NULL，因为这会额外的耗费mysql的处理时间来做优化。

mysql> explain select * from t_order where user_id=100 or user_id is null; +----+-------------+---------+-------------+---------------+---------+---------+-------+-------+-------------+ | id | select_type | table   | type        | possible_keys | key     | key_len | ref   | rows  | Extra       | +----+-------------+---------+-------------+---------------+---------+---------+-------+-------+-------------+ |  1 | SIMPLE      | t_order | ref_or_null | user_id       | user_id | 5       | const | 50325 | Using where | +----+-------------+---------+-------------+---------------+---------+---------+-------+-------+-------------+ 1 row in set (0.00 sec)

5.index_merge

经常出现在使用一张表中的多个索引时。mysql会将多个索引合并在一起，如下例:

mysql> explain select * from t_order where order_id=100 or user_id=10; +----+-------------+---------+-------------+-----------------+-----------------+---------+------+------+-------------------------------------------+ | id | select_type | table   | type        | possible_keys   | key             | key_len | ref  | rows | Extra                                     | +----+-------------+---------+-------------+-----------------+-----------------+---------+------+------+-------------------------------------------+ |  1 | SIMPLE      | t_order | index_merge | PRIMARY,user_id | PRIMARY,user_id | 4,5     | NULL |    2 | Using union(PRIMARY,user_id); Using where | +----+-------------+---------+-------------+-----------------+-----------------+---------+------+------+-------------------------------------------+ 1 row in set (0.09 sec)

6.unique_subquery

该联接类型用于替换value IN (SELECT primary_key FROM single_table WHERE some_expr)这样的子查询的ref。注意ref列，其中第二行显示的是func，表明unique_subquery是一个函数，而不是一个普通的ref。

mysql> explain select * from t_order where order_id in (select order_id from t_order where user_id=10); +----+--------------------+---------+-----------------+-----------------+---------+---------+------+--------+-------------+ | id | select_type        | table   | type            | possible_keys   | key     | key_len | ref  | rows   | Extra       | +----+--------------------+---------+-----------------+-----------------+---------+---------+------+--------+-------------+ |  1 | PRIMARY            | t_order | ALL             | NULL            | NULL    | NULL    | NULL | 100649 | Using where | |  2 | DEPENDENT SUBQUERY | t_order | unique_subquery | PRIMARY,user_id | PRIMARY | 4       | func |      1 | Using where | +----+--------------------+---------+-----------------+-----------------+---------+---------+------+--------+-------------+ 2 rows in set (0.00 sec)

7.index_subquery

该联接类型与上面的太像了，唯一的差别就是子查询查的不是主键而是非唯一索引。

mysql> explain select * from t_order where user_id in (select user_id from t_order where order_id>10); +----+--------------------+---------+----------------+-----------------+---------+---------+------+--------+--------------------------+ | id | select_type        | table   | type           | possible_keys   | key     | key_len | ref  | rows   | Extra                    | +----+--------------------+---------+----------------+-----------------+---------+---------+------+--------+--------------------------+ |  1 | PRIMARY            | t_order | ALL            | NULL            | NULL    | NULL    | NULL | 100649 | Using where              | |  2 | DEPENDENT SUBQUERY | t_order | index_subquery | PRIMARY,user_id | user_id | 5       | func |  50324 | Using index; Using where | +----+--------------------+---------+----------------+-----------------+---------+---------+------+--------+--------------------------+ 2 rows in set (0.00 sec)

8.range

按指定的范围进行检索，很常见。

mysql> explain select * from t_order where user_id in (100,200,300); +----+-------------+---------+-------+---------------+---------+---------+------+------+-------------+ | id | select_type | table   | type  | possible_keys | key     | key_len | ref  | rows | Extra       | +----+-------------+---------+-------+---------------+---------+---------+------+------+-------------+ |  1 | SIMPLE      | t_order | range | user_id       | user_id | 5       | NULL |    3 | Using where | +----+-------------+---------+-------+---------------+---------+---------+------+------+-------------+ 1 row in set (0.00 sec)

9.index

在进行统计时非常常见，此联接类型实际上会扫描索引树，仅比ALL快些。

mysql> explain select count(*) from t_order; +----+-------------+---------+-------+---------------+---------+---------+------+--------+-------------+ | id | select_type | table   | type  | possible_keys | key     | key_len | ref  | rows   | Extra       | +----+-------------+---------+-------+---------------+---------+---------+------+--------+-------------+ |  1 | SIMPLE      | t_order | index | NULL          | user_id | 5       | NULL | 100649 | Using index | +----+-------------+---------+-------+---------------+---------+---------+------+--------+-------------+ 1 row in set (0.00 sec)

10.ALL

完整的扫描全表，最慢的联接类型，尽可能的避免。

mysql> explain select * from t_order; +----+-------------+---------+------+---------------+------+---------+------+--------+-------+ | id | select_type | table   | type | possible_keys | key  | key_len | ref  | rows   | Extra | +----+-------------+---------+------+---------------+------+---------+------+--------+-------+ |  1 | SIMPLE      | t_order | ALL  | NULL          | NULL | NULL    | NULL | 100649 |       | +----+-------------+---------+------+---------------+------+---------+------+--------+-------+ 1 row in set (0.00 sec)

三.extra的说明

1.Distinct

MySQL发现第1个匹配行后,停止为当前的行组合搜索更多的行。对于此项没有找到合适的例子，求指点。

2.Not exists

因为b表中的order_id是主键，不可能为NULL，所以mysql在用a表的order_id扫描t_order表，并查找b表的行时，如果在b表发现一个匹配的行就不再继续扫描b了，因为b表中的order_id字段不可能为NULL。这样避免了对b表的多次扫描。

mysql> explain select count(1) from t_order a left join t_order_ext b on a.order_id=b.order_id where b.order_id is null;  +----+-------------+-------+-------+---------------+--------------+---------+-----------------+--------+--------------------------------------+ | id | select_type | table | type  | possible_keys | key          | key_len | ref             | rows   | Extra                                | +----+-------------+-------+-------+---------------+--------------+---------+-----------------+--------+--------------------------------------+ |  1 | SIMPLE      | a     | index | NULL          | express_type | 1       | NULL            | 100395 | Using index                          | |  1 | SIMPLE      | b     | ref   | order_id      | order_id     | 4       | test.a.order_id |      1 | Using where; Using index; Not exists | +----+-------------+-------+-------+---------------+--------------+---------+-----------------+--------+--------------------------------------+ 2 rows in set (0.01 sec)

3.Range checked for each record

这种情况是mysql没有发现好的索引可用，速度比没有索引要快得多。

mysql> explain select * from t_order t, t_order_ext s where s.order_id>=t.order_id and s.order_id<=t.order_id and t.express_type>5; +----+-------------+-------+-------+----------------------+--------------+---------+------+------+------------------------------------------------+ | id | select_type | table | type  | possible_keys        | key          | key_len | ref  | rows | Extra                                          | +----+-------------+-------+-------+----------------------+--------------+---------+------+------+------------------------------------------------+ |  1 | SIMPLE      | t     | range | PRIMARY,express_type | express_type | 1       | NULL |    1 | Using where                                    | |  1 | SIMPLE      | s     | ALL   | order_id             | NULL         | NULL    | NULL |    1 | Range checked for each record (index map: 0x1) | +----+-------------+-------+-------+----------------------+--------------+---------+------+------+------------------------------------------------+ 2 rows in set (0.00 sec)

4.Using filesort

在有排序子句的情况下很常见的一种情况。此时mysql会根据联接类型浏览所有符合条件的记录，并保存排序关键字和行指针，然后排序关键字并按顺序检索行。

mysql> explain select * from t_order order by express_type; +----+-------------+---------+------+---------------+------+---------+------+--------+----------------+ | id | select_type | table   | type | possible_keys | key  | key_len | ref  | rows   | Extra          | +----+-------------+---------+------+---------------+------+---------+------+--------+----------------+ |  1 | SIMPLE      | t_order | ALL  | NULL          | NULL | NULL    | NULL | 100395 | Using filesort | +----+-------------+---------+------+---------------+------+---------+------+--------+----------------+ 1 row in set (0.00 sec)

5.Using index

这是性能很高的一种情况。当查询所需的数据可以直接从索引树中检索到时，就会出现。上面的例子中有很多这样的例子，不再多举例了。

6.Using temporary

发生这种情况一般都是需要进行优化的。mysql需要创建一张临时表用来处理此类查询。

mysql> explain select * from t_order a left join t_order_ext b on a.order_id=b.order_id group by b.order_id; +----+-------------+-------+------+---------------+----------+---------+-----------------+--------+---------------------------------+ | id | select_type | table | type | possible_keys | key      | key_len | ref             | rows   | Extra                           | +----+-------------+-------+------+---------------+----------+---------+-----------------+--------+---------------------------------+ |  1 | SIMPLE      | a     | ALL  | NULL          | NULL     | NULL    | NULL            | 100395 | Using temporary; Using filesort | |  1 | SIMPLE      | b     | ref  | order_id      | order_id | 4       | test.a.order_id |      1 |                                 | +----+-------------+-------+------+---------------+----------+---------+-----------------+--------+---------------------------------+ 2 rows in set (0.00 sec)

7.Using where

当有where子句时，extra都会有说明。

8.Using sort_union(…)/Using union(…)/Using intersect(…)

下面的例子中user_id是一个检索范围，此时mysql会使用sort_union函数来进行索引的合并。而当user_id是一个固定值时，请参看上面type说明5.index_merge的例子，此时会使用union函数进行索引合并。

mysql> explain select * from t_order where order_id=100 or user_id>10; +----+-------------+---------+-------------+-----------------+-----------------+---------+------+------+------------------------------------------------+ | id | select_type | table   | type        | possible_keys   | key             | key_len | ref  | rows | Extra                                          | +----+-------------+---------+-------------+-----------------+-----------------+---------+------+------+------------------------------------------------+ |  1 | SIMPLE      | t_order | index_merge | PRIMARY,user_id | user_id,PRIMARY | 5,4     | NULL |    2 | Using sort_union(user_id,PRIMARY); Using where | +----+-------------+---------+-------------+-----------------+-----------------+---------+------+------+------------------------------------------------+ 1 row in set (0.00 sec)

对于Using intersect的例子可以参看下例，user_id与express_type发生了索引交叉合并。

mysql> explain select * from t_order where express_type=1 and user_id=100; +----+-------------+---------+-------------+----------------------+----------------------+---------+------+------+----------------------------------------------------+ | id | select_type | table   | type        | possible_keys        | key                  | key_len | ref  | rows | Extra                                              | +----+-------------+---------+-------------+----------------------+----------------------+---------+------+------+----------------------------------------------------+ |  1 | SIMPLE      | t_order | index_merge | user_id,express_type | user_id,express_type | 5,1     | NULL |    1 | Using intersect(user_id,express_type); Using where | +----+-------------+---------+-------------+----------------------+----------------------+---------+------+------+----------------------------------------------------+ 1 row in set (0.00 sec)

9.Using index for group-by

表明可以在索引中找到分组所需的所有数据，不需要查询实际的表。

mysql> explain select user_id from t_order group by user_id; +----+-------------+---------+-------+---------------+---------+---------+------+------+--------------------------+ | id | select_type | table   | type  | possible_keys | key     | key_len | ref  | rows | Extra                    | +----+-------------+---------+-------+---------------+---------+---------+------+------+--------------------------+ |  1 | SIMPLE      | t_order | range | NULL          | user_id | 5       | NULL |    3 | Using index for group-by | +----+-------------+---------+-------+---------------+---------+---------+------+------+--------------------------+ 1 row in set (0.00 sec)

除了上面的三个说明，还需要注意rows的数值，多行之间的数值是乘积的关系，可以估算大概要处理的行数，如果乘积很大，那就很有优化的必要了。

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