Question

在没有“filesort”和“using temporary”的情况下，有没有办法在MySQL中使用group by？尝试了不同的索引，不同的broup，不同的顺序但总是相同的结果 - “使用filesort”或“使用filesort +使用临时”。

CREATE TABLE IF NOT EXISTS `ank1` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `uid` int(11) NOT NULL DEFAULT '0',
  `kmk` int(3) NOT NULL,
  `order_by` int(11) NOT NULL,
  `gender` tinyint(1) NOT NULL,
  PRIMARY KEY (`id`),
  KEY `gender` (`gender`,`kmk`,`uid`,`order_by`)
) ENGINE=MyISAM  DEFAULT CHARSET=latin1 AUTO_INCREMENT=7 ;


mysql> explain SELECT gender, kmk, order_by FROM ank1
    -> where
    -> gender IN (1, 2, 3) and
    -> kmk IN (1, 2, 3, 12, 13, 23, 123)
    -> group by uid
    -> order by order_by DESC limit 0, 30;
+----+-------------+-------+-------+---------------+--------+---------+------+------+-----------------------------------------------------------+
| id | select_type | table | type  | possible_keys | key    | key_len | ref  | rows | Extra                                                     |
+----+-------------+-------+-------+---------------+--------+---------+------+------+-----------------------------------------------------------+
|  1 | SIMPLE      | ank1  | index | gender        | gender | 13      | NULL |    6 | Using where; Using index; Using temporary; Using filesort |
+----+-------------+-------+-------+---------------+--------+---------+------+------+-----------------------------------------------------------+
1 row in set (0.00 sec)

即使按同一列分组和排序也会给出相同的结果：

CREATE TABLE IF NOT EXISTS `ank1` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `uid` int(11) NOT NULL DEFAULT '0',
  `kmk` int(3) NOT NULL,
  `order_by` int(11) NOT NULL,
  `gender` tinyint(1) NOT NULL,
  PRIMARY KEY (`id`),
  KEY `gender` (`gender`,`kmk`,`order_by`)
) ENGINE=MyISAM  DEFAULT CHARSET=latin1 AUTO_INCREMENT=7 ;

mysql> explain SELECT gender, kmk, order_by FROM ank1
    -> where
    -> gender IN (1, 2, 3) and
    -> kmk IN (1, 2, 3, 12, 13, 23, 123)
    -> group by order_by
    -> order by order_by DESC limit 0, 30;
+----+-------------+-------+-------+---------------+--------+---------+------+------+-----------------------------------------------------------+
| id | select_type | table | type  | possible_keys | key    | key_len | ref  | rows | Extra                                                     |
+----+-------------+-------+-------+---------------+--------+---------+------+------+-----------------------------------------------------------+
|  1 | SIMPLE      | ank1  | index | gender        | gender | 9       | NULL |    6 | Using where; Using index; Using temporary; Using filesort |
+----+-------------+-------+-------+---------------+--------+---------+------+------+-----------------------------------------------------------+
1 row in set (0.00 sec)

MySQL - group by只能使用“filesort”和“using temporary”吗？

0 个答案: