我正在尝试加快在大约 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;
日期因查询而异。
这样做的目的是在添加日期或之前获取每个名称的最新条目。
当我对查询使用解释时,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 |
+----+-------------+------------------+-------+------------------+----------------------------------------------+---------+------+----------+-----------------------------------------------------------+
编辑: 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 |
+----+-------------+------------------+-------+---------------------------------------------------------------+----------------------------------------------+---------+------+----------+-------------------------------------------+
编辑: 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
编辑: 这是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 |
+----+-------------+--------------+-------+------------------------------------------------------------------------------------------+--------------------------+---------+--------------------------------------------------+------+---------------------------------------+