msyql中的order by null问题

如下：

mysql> select * from lry_name;
+----+------+------+
| id | name | addr |
+----+------+------+
|  1 | lry  | NULL |
|  2 | lry  | NULL |
|  5 | lry  | NULL |
|  7 | aaa  | NULL |
+----+------+------+

mysql> explain select sum(id) from lry_name group by name;
+----+-------------+----------+------------+------+---------------+------+---------+------+------+----------+---------------------------------+
| id | select_type | table    | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra                           |
+----+-------------+----------+------------+------+---------------+------+---------+------+------+----------+---------------------------------+
|  1 | SIMPLE      | lry_name | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    4 |   100.00 | Using temporary; Using filesort |
+----+-------------+----------+------------+------+---------------+------+---------+------+------+----------+---------------------------------+
1 row in set, 1 warning (0.00 sec)

mysql> explain select sum(id) from lry_name group by name order by null;
+----+-------------+----------+------------+------+---------------+------+---------+------+------+----------+-----------------+
| id | select_type | table    | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra           |
+----+-------------+----------+------------+------+---------------+------+---------+------+------+----------+-----------------+
|  1 | SIMPLE      | lry_name | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    4 |   100.00 | Using temporary |
+----+-------------+----------+------------+------+---------------+------+---------+------+------+----------+-----------------+
1 row in set, 1 warning (0.00 sec)

可以看到：

1）从上面的经验来看，ordery by null的作用是强制对查询结果禁用排序
2）如果在适应的场合没有必要对结果进行排序输入，那么我们可以用order by 取消排序，哪怕可能性能几乎没有提高，但是有的时候却可能有很大作用