作为开发者必须掌握的mysql操作sql语句优化-showprocesslistandexplain - 数据库编程

---------+---------+---------+-------+------+-------------+

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 | | 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:

作为开发者必须掌握的mysql操作sql语句优化-showprocesslistandexplain(四)