提高性能或重新设计“最大组” MySQL查询
作者：互联网
我正在使用MySQL5,目前有一个查询可向我获取所需的信息,但我认为它的性能可能会有所提高.
这是我建立的查询(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ORDER BY GROUP BY避免获取多个dom条目,以防万一’domcabling’的date_change为空,而另一个条目的日期与该’dom’相同.
我觉得我正在用火箭筒杀死一只老鼠.该查询需要3秒钟以上的时间,而“ livedata_dom”和“ livedata_domcabling”中的条目只有大约5k.另外,EXPLAIN告诉我使用了2个文件排序：
+----+-------------+------------+--------+-----------------------------+-----------------------------+---------+-----------------+------+----------------------------------------------+
| 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 |
+----+-------------+------------+--------+-----------------------------+-----------------------------+---------+-----------------+------+----------------------------------------------+
如何更改此查询以使其更有效？
使用伪数据(下面提供),这是预期的结果：
+-----+-------+---------+--------+----------+------------+-----------+---------------------+------+-----------+
| 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”条目都需要一行,
>最高date_change的“记录”记录
>如果记录不存在,则需要空字段
>一个条目每个dom可能有一个空的date_change字段(空的datetime字段被认为早于任何其他datetime)
>“集线器”的名称,当找到“ domcabling”条目时,否则为null
为3个表创建CREATE TABLE虚拟INSERT：
livedata_dom(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(大约10000个条目,并且增长缓慢)
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_hub(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');
解决方法:
尝试此重写(已在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)上的索引将有助于提高效率.
我不确定d.cluster =’localhost’的选择性(livedata_dom表中有多少行与该条件匹配？),但是在(集群)上添加索引也可能会有所帮助.
标签：performance,greatest-n-per-group,filesort,mysql 来源： https://codeday.me/bug/20191031/1978143.html