MySql 優化 group by 語句

 

默認狀況下，Mysql 對全部 group by 的字段進行排序，若是查詢包括 group by ，用戶想要避免排序結果的消耗。能夠指定 order by  null 禁止排序。

mysql> EXPLAIN select * from sys_log group by title;
+----+-------------+---------+------------+------+---------------+------+---------+------+-------+----------+---------------------------------+
| id | select_type | table   | partitions | type | possible_keys | key  | key_len | ref  | rows  | filtered | Extra                           |
+----+-------------+---------+------------+------+---------------+------+---------+------+-------+----------+---------------------------------+
|  1 | SIMPLE      | sys_log | NULL       | ALL  | NULL          | NULL | NULL    | NULL | 23733 |   100.00 | Using temporary; Using filesort |
+----+-------------+---------+------------+------+---------------+------+---------+------+-------+----------+---------------------------------+

 

mysql> EXPLAIN select * from sys_log group by title order by null;
+----+-------------+---------+------------+------+---------------+------+---------+------+-------+----------+-----------------+
| id | select_type | table   | partitions | type | possible_keys | key  | key_len | ref  | rows  | filtered | Extra           |
+----+-------------+---------+------------+------+---------------+------+---------+------+-------+----------+-----------------+
|  1 | SIMPLE      | sys_log | NULL       | ALL  | NULL          | NULL | NULL    | NULL | 23733 |   100.00 | Using temporary |
+----+-------------+---------+------------+------+---------------+------+---------+------+-------+----------+-----------------+

從上面的例子能夠看出，第一個查詢的 Extra 多了一個 filesort,因此查詢會較第二個查詢耗時。