(关于explain使用:比较好的例子)
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:
id | SELECT识别符。这是SELECT的查询序列号 |
select_type | SELECT类型,可以为以下任何一种:
|
table | 输出的行所引用的表 |
type | 联接类型。下面给出各种联接类型,按照从最佳类型到最坏类型进行排序:
|
possible_keys | 指出MySQL能使用哪个索引在该表中找到行 |
key | 显示MySQL实际决定使用的键(索引)。如果没有选择索引,键是NULL。 |
key_len | 显示MySQL决定使用的键长度。如果键是NULL,则长度为NULL。 |
ref | 显示使用哪个列或常数与key一起从表中选择行。 |
rows | 显示MySQL认为它执行查询时必须检查的行数。多行之间的数据相乘可以估算要处理的行数。 |
filtered | 显示了通过条件过滤出的行数的百分比估计值。 |
Extra | 该列包含MySQL解决查询的详细信息
|
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。