使用 MAX、WHERE 和 GROUP BY 在三个不同列上提高 SQL 查询速度

我正在尝试加快一个查询的速度，该查询大约需要 60 秒才能在大约 2000 万行的表上完成。

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

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

我试图运行的查询是：

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

日期在查询之间是可变的。

这样做的目的是获取每个名称在添加日期或之前的最新条目。

当我在查询上使用解释时，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 |
+----+-------------+------------------+-------+------------------+----------------------------------------------+---------+------+----------+-----------------------------------------------------------+

Edit: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 |
+----+-------------+------------------+-------+---------------------------------------------------------------+----------------------------------------------+---------+------+----------+-------------------------------------------+

Edit: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

Edit:这是HeavyE提供的答案的解释。

+----+-------------+--------------+-------+------------------------------------------------------------------------------------------+--------------------------+---------+--------------------------------------------------+------+---------------------------------------+
| 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 |
+----+-------------+--------------+-------+------------------------------------------------------------------------------------------+--------------------------+---------+--------------------------------------------------+------+---------------------------------------+

Stack Overflow 上有一篇很棒的文章，介绍了如何优化选择列中具有最大值的行：https://stackoverflow.com/a/7745635/633063 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;