mysql中explain
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2024-03-08 18:00:52
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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 | <union1,2> | 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 | <union2,3> | 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 | <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)
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。