Question

我正在“收藏”表（300万行）“items”表（600k行）之间进行连接。查询占用的时间从0.3秒到2秒，我希望我可以优化一些。

Favorites.faver_profile_id和Items.id已编入索引。我没有使用faver_profile_id索引，而是在（faver_profile_id，id）上创建了一个新索引，它消除了按id排序时所需的文件排序。不幸的是，这个索引根本没有帮助，我可能会删除它（是的，还有3个小时的停机时间才能删除索引..）

有关如何优化此查询的任何想法？

如果有帮助：
Favorite.removed和Item.removed在98％的时间都是“0” Favorite.collection_id大约80％的时间都是NULL。

SELECT `Item`.`id`, `Item`.`source_image`, `Item`.`cached_image`, `Item`.`source_title`, `Item`.`source_url`, `Item`.`width`, `Item`.`height`, `Item`.`fave_count`, `Item`.`created`        
FROM `favorites` AS `Favorite`
LEFT JOIN `items` AS `Item` 
ON (`Item`.`removed` = 0 AND `Favorite`.`notice_id` = `Item`.`id`) 
WHERE ((`faver_profile_id` = 1) AND (`collection_id` IS NULL) AND (`Favorite`.`removed` = 0) AND (`Item`.`removed` = '0')) 
ORDER BY `Favorite`.`id` desc LIMIT 50;

+----+-------------+----------+--------+----------------------------------------------------- ----------+------------------+---------+-----------------------------------------+------+-------------+
| id | select_type | table    | type   | possible_keys                                                 | key              | key_len | ref                                     | rows | Extra       |
+----+-------------+----------+--------+---------------------------------------------------------------+------------------+---------+-----------------------------------------+------+-------------+
|  1 | SIMPLE      | Favorite | ref    | notice_id,faver_profile_id,collection_id_idx,idx_faver_idx_id | idx_faver_idx_id |       4 | const                                   | 7910 | Using where |
|  1 | SIMPLE      | Item     | eq_ref | PRIMARY                                                       | PRIMARY          |       4 | gragland_imgfavebeta.Favorite.notice_id |    1 | Using where |
+----+-------------+----------+--------+---------------------------------------------------------------+------------------+---------+-----------------------------------------+------+-------------+

+-----------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table     | Create Table                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
+-----------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| favorites | CREATE TABLE `favorites` (
             `id` int(11) NOT NULL auto_increment COMMENT 'unique identifier',
             `faver_profile_id` int(11) NOT NULL default '0',
             `collection_id` int(11) default NULL,
             `collection_order` int(8) default NULL,
             `created` datetime NOT NULL default '0000-00-00 00:00:00' COMMENT 'date this record was created',
             `modified` timestamp NOT NULL default CURRENT_TIMESTAMP on update CURRENT_TIMESTAMP COMMENT 'date this record was modified',
             `notice_id` int(11) NOT NULL default '0',
             `removed` tinyint(1) NOT NULL default '0',
              PRIMARY KEY  (`id`),
              KEY `notice_id` (`notice_id`),
              KEY `faver_profile_id` (`faver_profile_id`),
              KEY `collection_id_idx` (`collection_id`),
              KEY `idx_faver_idx_id` (`faver_profile_id`,`id`)
              ) ENGINE=InnoDB DEFAULT CHARSET=latin1 |
+-----------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+


+-------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
+-------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| items |CREATE TABLE `items` (
         `id` int(11) NOT NULL auto_increment COMMENT 'unique identifier',
         `submitter_id` int(11) NOT NULL default '0' COMMENT 'who made the update',
         `source_image` varchar(255) default NULL COMMENT 'update content',
         `cached_image` varchar(255) default NULL,
         `source_title` varchar(255) NOT NULL default '',
         `source_url` text NOT NULL,
         `width` int(4) NOT NULL default '0',
         `height` int(4) NOT NULL default '0',
         `status` varchar(122) NOT NULL default '',
         `popular` int(1) NOT NULL default '0',
         `made_popular` timestamp NULL default NULL,
         `fave_count` int(9) NOT NULL default '0',
         `tags` text,
         `user_art` tinyint(1) NOT NULL default '0',
         `nudity` tinyint(1) NOT NULL default '0',
         `created` datetime NOT NULL default '0000-00-00 00:00:00' COMMENT 'date this record was created',
         `modified` timestamp NOT NULL default CURRENT_TIMESTAMP on update CURRENT_TIMESTAMP COMMENT 'date this record was modified',
         `removed` int(1) NOT NULL default '0',
         `nofront` tinyint(1) NOT NULL default '0',
         `test` varchar(10) NOT NULL default '',
         `recs` text,
         `recs_data` text,
         PRIMARY KEY  (`id`),
         KEY `notice_profile_id_idx` (`submitter_id`),
         KEY `content` (`source_image`),
         KEY `idx_popular` (`popular`),
         KEY `idx_madepopular` (`made_popular`),
         KEY `idx_favecount_idx_id` (`fave_count`,`id`)
         ) ENGINE=InnoDB DEFAULT CHARSET=latin1 |
+-------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+

Answer 1

首先，您通过favorites.id订购，这是favorites表中的聚簇主键。您无需加入favorites加入items而非items加入favorites。

其次，WHERE中的（Item。removed ='0'）过多，因为JOIN中已经使用了相同的条件。

第三，将连接中的条件顺序更改为：

`Favorite`.`notice_id` = `Item`.`id` AND `Item`.`removed` = 0

优化器将能够使用主键作为索引。您甚至可以考虑在items表上创建（id，removed）索引。

接下来，在favorites中创建（faver_profile_id，删除）索引（或更好地更新faver_profile_id索引）并将WHERE中的条件顺序更改为以下内容：

(`faver_profile_id` = 1)
AND (`Favorite`.`removed` = 0)
AND (`collection_id` IS NULL)

UPD ：很抱歉，我错过了您已加入favorites items。然后不需要ORDER BY。您应该得到如下内容：

SELECT
    `Item`.`id`,
    `Item`.`source_image`,
    `Item`.`cached_image`,
    `Item`.`source_title`,
    `Item`.`source_url`, 
    `Item`.`width`,
    `Item`.`height`,
    `Item`.`fave_count`,
    `Item`.`created`        
FROM `favorites` AS `Favorite`
LEFT JOIN `items` AS `Item` 
ON (`Favorite`.`notice_id` = `Item`.`id` AND `Item`.`removed` = 0) 
WHERE `faver_profile_id` = 1
    AND `Favorite`.`removed` = 0
    AND `collection_id` IS NULL
LIMIT 50;

还有一件事，当你有KEY idx_faver_idx_id（faver_profile_id，id）时，你不需要KEY faver_profile_id（faver_profile_id），因为第二个索引只复制idx_faver_idx_id的一半。我希望你能像我建议的那样扩展第二个索引。

Answer 2

从备份中获取表的副本，并尝试在Favorite表上创建一个索引，涵盖所有WHERE和JOIN条件，即（removed，collection_id，profile_id）。对Item也一样。它可能会有所帮助，但会使插入更慢。

如果由于约束仍然需要进行全表扫描，SQL引擎将不会使用索引，是吗？

需要帮助通过join优化MYSQL查询

2 个答案: