mysql - 提高性能或重新设计'每组最大'的mysql查询

我正在使用mysql5，目前我有一个查询，可以得到我需要的信息，但我觉得它可以在性能方面得到改进。
以下是我构建的查询（大致遵循this guide）：

SELECT d.*, dc.date_change, dc.cwd, h.name as hub
FROM livedata_dom AS d
      LEFT JOIN ( SELECT dc1.*
        FROM livedata_domcabling as dc1
        LEFT JOIN livedata_domcabling AS dc2
        ON dc1.dom_id = dc2.dom_id AND dc1.date_change < dc2.date_change
        WHERE dc2.dom_id IS NULL
        ORDER BY dc1.date_change desc) AS dc ON (d.id = dc.dom_id)
      LEFT JOIN livedata_hub AS h ON (d.id = dc.dom_id AND dc.hub_id = h.id)
WHERE d.cluster = 'localhost'
GROUP BY d.id;

编辑：使用ORDERBY+GROUPBY避免在“domcling”中有一个日期更改为空的条目和另一个日期为同一“dom”的条目时获取多个dom条目。
我觉得我在用火箭筒杀老鼠。此查询需要3秒以上的时间，在“livedata\u dom”和“livedata\u domconlineing”中只有大约5K个条目。另外，explain告诉我使用了两个文件队列：

+----+-------------+------------+--------+-----------------------------+-----------------------------+---------+-----------------+------+----------------------------------------------+
| id | select_type | table      | type   | possible_keys               | key                         | key_len | ref             | rows | Extra                                        |
+----+-------------+------------+--------+-----------------------------+-----------------------------+---------+-----------------+------+----------------------------------------------+
|  1 | PRIMARY     | d          | ALL    | NULL                        | NULL                        | NULL    | NULL            |    3 | Using where; Using temporary; Using filesort |
|  1 | PRIMARY     | <derived2> | ALL    | NULL                        | NULL                        | NULL    | NULL            |    3 |                                              |
|  1 | PRIMARY     | h          | eq_ref | PRIMARY                     | PRIMARY                     | 4       | dc.hub_id       |    1 |                                              |
|  2 | DERIVED     | dc1        | ALL    | NULL                        | NULL                        | NULL    | NULL            |    4 | Using filesort                               |
|  2 | DERIVED     | dc2        | ref    | livedata_domcabling_dc592d9 | livedata_domcabling_dc592d9 | 4       | live.dc1.dom_id |    2 | Using where; Not exists                      |
+----+-------------+------------+--------+-----------------------------+-----------------------------+---------+-----------------+------+----------------------------------------------+

如何更改此查询以提高效率？
使用虚拟数据（如下所示），这是预期结果：

+-----+-------+---------+--------+----------+------------+-----------+---------------------+------+-----------+
| id  | mb_id | prod_id | string | position | name       | cluster   | date_change         | cwd  | hub       |
+-----+-------+---------+--------+----------+------------+-----------+---------------------+------+-----------+
| 249 | 47    | 47      |     47 |       47 | SuperDOM47 | localhost | NULL                | NULL | NULL      |
| 250 | 48    | 48      |     48 |       48 | SuperDOM48 | localhost | 2014-04-16 05:23:00 | 32A  | megahub01 |
| 251 | 49    | 49      |     49 |       49 | SuperDOM49 | localhost | NULL                | 22B  | megahub01 |
+-----+-------+---------+--------+----------+------------+-----------+---------------------+------+-----------+

基本上，每个“dom”条目需要一行
更改日期最高的“DomCabling”记录
如果记录不存在，则需要空字段
每个dom的一个条目可能有一个空的date_change字段（空的datetime字段被认为比任何其他datetime都旧）
“集线器”的名称，当找到“domCabling”条目时，否则为空
为3个表创建table+dummy insert：
livedata_dom（大约5000个条目）

CREATE TABLE `livedata_dom` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `mb_id` varchar(12) NOT NULL,
  `prod_id` varchar(8) NOT NULL,
  `string` int(11) NOT NULL,
  `position` int(11) NOT NULL,
  `name` varchar(30) NOT NULL,
  `cluster` varchar(9) NOT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `mb_id` (`mb_id`),
  UNIQUE KEY `prod_id` (`prod_id`),
  UNIQUE KEY `name` (`name`),
  UNIQUE KEY `livedata_domgood_string_7bff074107b0e5a0_uniq` (`string`,`position`,`cluster`)
) ENGINE=InnoDB AUTO_INCREMENT=5485 DEFAULT CHARSET=latin1;

INSERT INTO `livedata_dom` VALUES (251,'49','49',49,49,'SuperDOM49','localhost'),(250,'48','48',48,48,'SuperDOM48','localhost'),(249,'47','47',47,47,'SuperDOM47','localhost');

LiveData_-DomCabling（约10000个条目，增长缓慢）

CREATE TABLE `livedata_domcabling` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `dom_id` int(11) NOT NULL,
  `hub_id` int(11) NOT NULL,
  `cwd` varchar(3) NOT NULL,
  `date_change` datetime DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `livedata_domcabling_dc592d9` (`dom_id`),
  KEY `livedata_domcabling_4366aa6e` (`hub_id`),
  CONSTRAINT `dom_id_refs_id_73e89ce0c50bf0a6` FOREIGN KEY (`dom_id`) REFERENCES `livedata_dom` (`id`),
  CONSTRAINT `hub_id_refs_id_179c89d8bfd74cdf` FOREIGN KEY (`hub_id`) REFERENCES `livedata_hub` (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=5397 DEFAULT CHARSET=latin1;

INSERT INTO `livedata_domcabling` VALUES (1,251,1,'22B',NULL),(2,250,1,'33A',NULL),(6,250,1,'32A','2014-04-16 05:23:00'),(5,250,1,'22B','2013-05-22 00:00:00');

LiveData_集线器（大约100个条目）

CREATE TABLE `livedata_hub` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(14) NOT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `name` (`name`)
) ENGINE=InnoDB AUTO_INCREMENT=98 DEFAULT CHARSET=latin;

INSERT INTO `livedata_hub` VALUES (1,'megahub01');

最佳答案

尝试重写（在SQL-Fiddle中测试：

SELECT
    d.*, dc.date_change, dc.cwd, h.name as hub
FROM
    livedata_dom AS d
  LEFT JOIN
    livedata_domcabling as dc
        ON dc.id =
           ( SELECT id
             FROM livedata_domcabling AS dcc
             WHERE dcc.dom_id = d.id
             ORDER BY date_change DESC
               LIMIT 1
          )
  LEFT JOIN
    livedata_hub AS h
        ON dc.hub_id = h.id
  WHERE
     d.cluster = 'localhost' ;

而(dom_id, date_change)指数有助于提高效率。
我不确定d.cluster = 'localhost'的选择性（表中有多少行与此条件匹配？）但在livedata_dom上添加索引也可能有帮助。