_keys | key | key_len | ref | rows | Extra |
+----+--------------------+-------+-------+-------------------+---------+---------+-------+------+--------------------------+
| 1 | PRIMARY | t3 | index | NULL | PRIMARY | 4 | NULL | 1000 | Using where; Using index |
| 2 | DEPENDENT SUBQUERY | t3 | const | PRIMARY,idx_t3_id | PRIMARY | 4 | const | 1 | Using index |
+----+--------------------+-------+-------+-------------------+---------+---------+-------+------+--------------------------+
(7).DERIVED
派生表的SELECT(FROM子句的子查询)
mysql> explain select * from (select * from t3 where id=3952602) a ;
+----+-------------+------------+--------+-------------------+---------+---------+------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+------------+--------+-------------------+---------+---------+------+------+-------+
| 1 | PRIMARY | | system | NULL | NULL | NULL | NULL | 1 | |
| 2 | DERIVED | t3 | const | PRIMARY,idx_t3_id | PRIMARY | 4 | | 1 | |
+----+-------------+------------+--------+-------------------+---------+---------+------+------+-------+
3.table
显示这一行的数据是关于哪张表的.
有时不是真实的表名字,看到的是derivedx(x是个数字,我的理解是第几步执行的结果)
mysql> explain select * from (select * from ( select * from t3 where id=3952602) a) b;
+----+-------------+------------+--------+-------------------+---------+---------+------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+------------+--------+-------------------+---------+---------+------+------+-------+
| 1 | PRIMARY | | system | NULL | NULL | NULL | NULL | 1 | |
| 2 | DERIVED | | system | NULL | NULL | NULL | NULL | 1 | |
| 3 | DERIVED | t3 | const | PRIMARY,idx_t3_id | PRIMARY | 4 | | 1 | |
+----+-------------+------------+--------+-------------------+---------+---------+------+------+-------+
4.type
这列很重要,显示了连接使用了哪种类别,有无使用索引.
从最好到最差的连接类型为const、eq_reg、ref、range、indexhe和ALL
(1).system
这是const联接类型的一个特例。表仅有一行满足条件.如下(t3表上的id是primary key)
mysql> explain select * from (select * from t3 where id=3952602) a ;
+----+-------------+------------+--------+-------------------+---------+---------+------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+------------+--------+-------------------+---------+---------+------+------+-------+
| 1 | PRIMARY | | system | NULL | NULL | NULL | NULL | 1 | |
| 2 | DERIVED | t3 | const | PRIMARY,idx_t3_id | PRIMARY | 4 | | 1 | |
+----+-------------+------------+--------+-------------------+---------+---------+------+------+-------+
(2).const
表最多有一个匹配行，它将在查询开始时被读取。因为仅有一行，在这行的列值可被优化器剩余部分认为是常数。const表很快，因为它们只读取一次！
const用于用常数值比较PRIMARY KEY或UNIQUE索引的所有部分时。在下面的查询中，tbl_name可以用于const表：
SELECT * from tbl_name WHERE primary_key=1；
SELECT * from tbl_name WHERE primary_key_part1=1和primary_key_part2=2 |