我目前加入 5 个表以选择 20 个对象来显示用户，不幸的是，如果我使用 GROUP BY 和 ORDER BY 它会变得非常慢。

示例查询如下所示:

SELECT r.name, l.name, o.typ, o.id, persons, children, description, rating, totalratings, minprice, picture FROM angebote as a 
JOIN objekte as o ON a.fid_objekt = o.id 
JOIN regionen as r ON a.fid_region = r.id 
JOIN laender as l ON a.fid_land = l.id 
WHERE l.slug="aegypten" AND a.letztes_angebot >= 1 
GROUP BY a.fid_objekt ORDER BY rating DESC LIMIT 0,20 

查询的 EXPLAIN 显示了这一点:

+------+-------------+-------+--------+----------------------------+------------+---------+---------------------------------------+--------+--------------------------------------------------------+
| id   | select_type | table | type   | possible_keys              | key        | key_len | ref                                   | rows   | Extra                                                  |
+------+-------------+-------+--------+----------------------------+------------+---------+---------------------------------------+--------+--------------------------------------------------------+
|    1 | SIMPLE      | l     | ref    | PRIMARY,slug               | slug       | 767     | const                                 |      1 | Using index condition; Using temporary; Using filesort |
|    1 | SIMPLE      | o     | ALL    | PRIMARY                    | NULL       | NULL    | NULL                                  | 186779 | Using join buffer (flat, BNL join)                     |
|    1 | SIMPLE      | a     | ref    | unique_key,letztes_angebot | unique_key | 8       | ferienhaeuser.o.id,ferienhaeuser.l.id |      1 | Using where                                            |
|    1 | SIMPLE      | r     | eq_ref | PRIMARY                    | PRIMARY    | 4       | ferienhaeuser.a.fid_region            |      1 |                                                        |
+------+-------------+-------+--------+----------------------------+------------+---------+---------------------------------------+--------+--------------------------------------------------------+

所以看起来它没有为表 objekte 使用键，Profiling 说它使用 2.7s 来复制到 tmp 表。

我尝试使用 (SELECT * GROUP BY id) 而不是 FROM angebote 或 JOIN objekte 但不幸的是这并没有改善。

用于WHERE、ORDER BY 和GROUP BY 的字段也被编入索引。

我想我在这里错过了一些基本概念，我们将不胜感激。

因为很可能我在表格上犯了错误，这里是对它们的描述:

对象

+---------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| objekte | CREATE TABLE `objekte` (
  `id` int(11) NOT NULL,
  `typ` varchar(50) NOT NULL,
  `persons` int(11) NOT NULL,
  `children` int(11) NOT NULL,
  `description` text NOT NULL,
  `rating` float NOT NULL,
  `totalratings` int(11) NOT NULL,
  `minprice` float NOT NULL,
  `picture` varchar(255) NOT NULL,
  `last_offer` int(11) NOT NULL,
  PRIMARY KEY (`id`),
  KEY `minprice` (`minprice`),
  KEY `rating` (`rating`),
  KEY `last_offer` (`last_offer`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 |
+---------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+

安格博特

    +-----------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| angebote | CREATE TABLE `angebote` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `fid_objekt` int(11) NOT NULL,
  `fid_land` int(11) NOT NULL,
  `fid_region` int(11) NOT NULL,
  `fid_subregion` int(11) NOT NULL,
  `letztes_angebot` int(11) NOT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `unique_key` (`fid_objekt`,`fid_land`,`fid_region`,`fid_subregion`),
  KEY `letztes_angebot` (`letztes_angebot`),
  KEY `fid_objekt` (`fid_objekt`),
  KEY `fid_land` (`fid_land`),
  KEY `fid_region` (`fid_region`),
  KEY `fid_subregion` (`fid_subregion`)
) ENGINE=InnoDB AUTO_INCREMENT=2433073 DEFAULT CHARSET=utf8 |
+-----------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+

laender, regionen, subregionen(相同结构)

+---------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| laender | CREATE TABLE `laender` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `iso` varchar(2) NOT NULL,
  `name` varchar(255) NOT NULL,
  `slug` varchar(255) NOT NULL,
  `letztes_angebot` int(11) NOT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `iso` (`iso`),
  KEY `slug` (`slug`),
  KEY `letztes_angebot` (`letztes_angebot`)
) ENGINE=InnoDB AUTO_INCREMENT=107 DEFAULT CHARSET=utf8 |
+---------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+

最佳答案

首先这是一个非标准的group by。因此，当您升级到 mysql 5.7 时它将停止工作。

最大的问题是 objekte 表没有使用索引。更糟糕的是，您在该表的评级字段上进行排序，但索引仍未被使用。一个可能的解决方案是像这样创建一个复合索引:

CREATE INDEX objekte_idx ON objekte(id,rating);

关于mysql - 慢速 MySQL 查询，按限制排序，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/40608097/