Question

我正在尝试加速在大约2000万行的表上完成大约需要60秒的查询。

对于此示例，该表有三列（id，dateAdded，name）。 id是主键。我添加到表中的索引是：

(dateAdded)
(name)
(id, name)
(id, name, dateAdded)

我尝试运行的查询是：

SELECT MAX(id) as id, name 
FROM exampletable 
WHERE dateAdded <= '2014-01-20 12:00:00' 
GROUP BY name 
ORDER BY NULL;

日期从查询变为查询。

这样做的目的是在添加日期之前或之前获取每个名称的最新条目。

当我在查询中使用explain时，它告诉我它正在使用（id，name，dateAdded）索引。

+----+-------------+------------------+-------+------------------+----------------------------------------------+---------+------+----------+-----------------------------------------------------------+
| id | select_type | table            | type  | possible_keys    | key                                          | key_len | ref  | rows     | Extra                                                     |
+----+-------------+------------------+-------+------------------+----------------------------------------------+---------+------+----------+-----------------------------------------------------------+
|  1 | SIMPLE      | exampletable     | index | date_added_index | id_element_name_date_added_index             | 162     | NULL | 22016957 | Using where; Using index; Using temporary; Using filesort |
+----+-------------+------------------+-------+------------------+----------------------------------------------+---------+------+----------+-----------------------------------------------------------+

修改从评论中添加了两个新索引：

(dateAdded, name, id)
(name, id)

+----+-------------+------------------+-------+---------------------------------------------------------------+----------------------------------------------+---------+------+----------+-------------------------------------------+
| id | select_type | table            | type  | possible_keys                                                 | key                                          | key_len | ref  | rows     | Extra                                     |
+----+-------------+------------------+-------+---------------------------------------------------------------+----------------------------------------------+---------+------+----------+-------------------------------------------+
|  1 | SIMPLE      | exampletable     | index | date_added_index,date_added_name_id_index                     | id__name_date_added_index                    | 162     | NULL | 22040469 | Using where; Using index; Using temporary |
+----+-------------+------------------+-------+---------------------------------------------------------------+----------------------------------------------+---------+------+----------+-------------------------------------------+

修改添加了创建表脚本。

CREATE TABLE `exampletable` (
  `id` int(10) NOT NULL auto_increment,
  `dateAdded` timestamp NULL default CURRENT_TIMESTAMP,
  `name` varchar(50) character set utf8 default '',
  PRIMARY KEY  (`id`),
  KEY `date_added_index` (`dateAdded`),
  KEY `name_index` USING BTREE (`name`),
  KEY `id_name_index` USING BTREE (`id`,`name`),
  KEY `id_name_date_added_index` USING BTREE (`id`,`dateAdded`,`name`),
  KEY `date_added_name_id_index` USING BTREE (`dateAdded`,`name`,`id`),
  KEY `name_id_index` USING BTREE (`name`,`id`)
) ENGINE=MyISAM AUTO_INCREMENT=22046064 DEFAULT CHARSET=latin1

修改以下是HeavyE提供的答案中的解释。

+----+-------------+--------------+-------+------------------------------------------------------------------------------------------+--------------------------+---------+--------------------------------------------------+------+---------------------------------------+
| id | select_type | table        | type  | possible_k                                                                               | key                      | key_len | ref                                              | rows | Extra                                 |
+----+-------------+--------------+-------+------------------------------------------------------------------------------------------+--------------------------+---------+--------------------------------------------------+------+---------------------------------------+
|  1 | PRIMARY     | <derived2>   | ALL   | NULL                                                                                     | NULL                     | NULL    | NULL                                             | 1732 | Using temporary; Using filesort       |
|  1 | PRIMARY     | example1     | ref   | date_added_index,name_index,date_added_name_id_index,name_id_index,name_date_added_index | date_added_name_id_index | 158     | maxDateByElement.dateAdded,maxDateByElement.name |    1 | Using where; Using index              |
|  2 | DERIVED     | exampletable | range | date_added_index,date_added_name_id_index                                                | name_date_added_index    | 158     | NULL                                             | 1743 | Using where; Using index for group-by |
+----+-------------+--------------+-------+------------------------------------------------------------------------------------------+--------------------------+---------+--------------------------------------------------+------+---------------------------------------+

Answer 1

有一个很棒的Stack Overflow帖子，用于优化选择列中包含最大值的行：https://stackoverflow.com/a/7745635/633063

这看起来有点乱，但效果很好：

SELECT example1.name, MAX(example1.id)
FROM exampletable example1
INNER JOIN (
select name, max(dateAdded) dateAdded
from exampletable
where dateAdded  <= '2014-01-20 12:00:00' 
group by name
) maxDateByElement on example1.name = maxDateByElement.name AND example1.dateAdded = maxDateByElement.dateAdded
GROUP BY name;

Answer 2

为什么你在许多键上使用索引？如果你的where子句只包含一个列，那么只使用那个索引，将index添加到dateAdded和name上，然后在sql语句中使用，如下所示：

SELECT MAX(id) as id, name 
FROM exampletable 
USE INDEX (dateAdded_index) USE INDEX FOR GROUP BY (name_index) 
WHERE dateAdded <= '2014-01-20 12:00:00' 
GROUP BY name
ORDER BY NULL;

如果你想了解更多信息，请点击link。请告诉我，是否给出了一些积极的结果。

Answer 3

如果where命令没有区别，那么它的最大值（id）或名称。我会通过完全消除Max（id）来测试索引，并查看按名称分组是否很快。然后我会添加Min（id）以查看它是否比Max（id）更快。（我看到这有所不同）。

此外，您应该通过NULL测试顺序。尝试按名称desc订购，或按名称订购asc。 Clark Vera

使用MAX，WHERE和GROUP BY在三个不同的列上提高SQL查询的速度

3 个答案: