mysql explain

转自:http://blog.chinaunix.net/uid-540802-id-3419311.html

explain显示了mysql如何使用索引来处理select语句以及连接表。可以帮助选择更好的索引和写出更优化的查询语句。

使用方法,在select语句前加上explain就可以了:

如:

explain select surname,first_name form a,b where a.id=b.id 

EXPLAIN列的解释

实例

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:

select_type的说明

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 | <union1,2> | ALL   | NULL          | NULL    | NULL    | NULL  | NULL |       | 
    +----+--------------+------------+-------+---------------+---------+---------+-------+------+-------+ 
    3 rows in set (0.34 sec) 

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       | <union2,3> | ALL   | NULL          | NULL    | NULL    | NULL  |   NULL |             | 
    +----+--------------------+------------+-------+---------------+---------+---------+-------+--------+-------------+ 
    4 rows in set (0.03 sec) 

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) 

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     | <derived2> | system | NULL          | NULL    | NULL    | NULL |    1 |             | 
    |  2 | DERIVED     | t_order    | const  | PRIMARY       | PRIMARY | 4       |      |    1 | Using index | 
    +----+-------------+------------+--------+---------------+---------+---------+------+------+-------------+ 
    2 rows in set (0.03 sec) 

type的说明

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。

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) 

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) 

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) 

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) 

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) 

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) 

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) 

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) 

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) 

extra的说明

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。

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) 

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)

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) 

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。

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) 

当有where子句时,extra都会有说明。

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) 

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。