explain命令如下:
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
其中select_type列指明该条SQL的读取操作的操作类型。
select_type共有六种类型:simple、primmy、subQuery、derived、union、union result。
1、simple
表示该条sql是简单的select,不包含任何子查询和union,例:
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
2、primmy
查询中如果包含了任何一个子查询,最外层的查询就会被标记为primmy,例:
mysql> explain select * from t_blog where id = (select id from t_type where name = "JAVA");
+----+-------------+--------+-------+---------------+---------+---------+-------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------+-------+---------------+---------+---------+-------+------+-------------+
| 1 | PRIMARY | t_blog | const | PRIMARY | PRIMARY | 4 | const | 1 | |
| 2 | SUBQUERY | t_type | ALL | NULL | NULL | NULL | NULL | 4 | Using where |
+----+-------------+--------+-------+---------------+---------+---------+-------+------+-------------+
2 rows in set
这条sql一共读取了两张表,t_type作为子查询被加载,t_blog作为最外部的读取操作,被标记为 PRIMMY。
3、subquery
查询中,在select或where自居中包含了子查询,该子查询就会被标记为subquery,如上例的t_type
4、derived
在from里列表中包含了子查询,该子查询会被标记为derived(衍生),例:
mysql> explain select * from t_blog inner join (select id from t_type) a on t_blog.typeId = a.id;
+----+-------------+------------+-------+---------------+---------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+------------+-------+---------------+---------+---------+------+------+-------------+
| 1 | PRIMARY | <derived2> | ALL | NULL | NULL | NULL | NULL | 4 | |
| 1 | PRIMARY | t_blog | ALL | NULL | NULL | NULL | NULL | 7 | Using where |
| 2 | DERIVED | t_type | index | NULL | PRIMARY | 4 | NULL | 4 | Using index |
+----+-------------+------------+-------+---------------+---------+---------+------+------+-------------+
3 rows in set
t_type表读取时,出现在from语句中,因此被标记为derived。mysql会先将from语句的子查询的查询结果放到一张临时表中,然后再将最终的结果返回,所以,是很耗费内存的一种操作。值得注意的是,在第一行的table列出现了<derived2>字样,表示该表是个衍生虚表,他的来源根据后面的数字“2”来寻找,这个“2”就是第一列的id,<derived2>表示是id为2的衍生表的衍生虚表。
5、union
若sql中包含了union,第二个select会被标记为union
mysql> explain select * from t_blog b left join t_type t on b.typeId = t.id
union
select * from t_blog b1 right join t_type t1 on b1.typeId = t1.id;
+------+--------------+------------+--------+---------------+---------+---------+---------------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+------+--------------+------------+--------+---------------+---------+---------+---------------+------+-------+
| 1 | PRIMARY | b | ALL | NULL | NULL | NULL | NULL | 7 | |
| 1 | PRIMARY | t | eq_ref | PRIMARY | PRIMARY | 4 | blog.b.typeId | 1 | |
| 2 | UNION | t1 | ALL | NULL | NULL | NULL | NULL | 4 | |
| 2 | UNION | b1 | ALL | NULL | NULL | NULL | NULL | 7 | |
| NULL | UNION RESULT | <union1,2> | ALL | NULL | NULL | NULL | NULL | NULL | |
+------+--------------+------------+--------+---------------+---------+---------+---------------+------+-------+
5 rows in set
t1表和t2表都出现在union后的select,因此都被标及为union
6、union result
表示该查询是从union表中获取结果的select,是union的结果集,如上例,上条select就是获取的union的结果集