带顺序的MySQL查询太慢
作者：互联网
我知道以前曾问过这种问题,但是没有一个问题使我找到了解决方案.
这是我的查询：
SELECT p.photoid, c.comment, p.path, p.smallfile, p.bigfile, c.userid, c.dateposted, u.username, c.likephoto
FROM bs_photocomments c, photos p, user u
WHERE c.active = 1
AND u.userid = c.userid
AND p.photoid = c.photoid
AND c.id = (SELECT ID FROM bs_photocomments WHERE photoid = p.photoid ORDER BY ID DESC LIMIT 1)
ORDER BY c.id DESC LIMIT 2
关于最后一个AND的一些解释：
基本上,我正在尝试获取20张独特照片的最新评论.例如,如果一张照片有5条评论,我只希望显示最后一条.
ORDER BY c.id是杀手..只有18万条评论和10万张照片.如果我删除该ORDER BY,查询会立即返回,但显然不会返回我想要的结果.
以下是索引：
+------------------+------------+--------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment |
+------------------+------------+--------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| bs_photocomments | 0 | PRIMARY | 1 | id | A | 183269 | NULL | NULL | | BTREE | |
| bs_photocomments | 1 | photocomments_idx1 | 1 | photoid | A | 183269 | NULL | NULL | YES | BTREE | |
| bs_photocomments | 1 | photocomments_idx2 | 1 | active | A | 4 | NULL | NULL | YES | BTREE | |
| bs_photocomments | 1 | photocomments_idx3 | 1 | dateposted | A | 183269 | NULL | NULL | YES | BTREE | |
| bs_photocomments | 1 | photocomments_idx4 | 1 | userid | A | 4953 | NULL | NULL | YES | BTREE | |
| bs_photocomments | 1 | photocomments_idx5 | 1 | puserid | A | 4072 | NULL | NULL | YES | BTREE | |
| bs_photocomments | 1 | photocomments_idx6 | 1 | id | A | 183269 | NULL | NULL | | BTREE | |
| bs_photocomments | 1 | photocomments_idx6 | 2 | photoid | A | 183269 | NULL | NULL | YES | BTREE | |
| bs_photocomments | 1 | photocomments_idx7 | 1 | photoid | A | 183269 | NULL | NULL | YES | BTREE | |
| bs_photocomments | 1 | photocomments_idx7 | 2 | userid | A | 183269 | NULL | NULL | YES | BTREE | |
+------------------+------------+--------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
+--------+------------+--------------+--------------+--------------+-----------+-------------+----------+--------+------+------------+---------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment |
+--------+------------+--------------+--------------+--------------+-----------+-------------+----------+--------+------+------------+---------+
| photos | 0 | PRIMARY | 1 | photoid | A | 57736 | NULL | NULL | | BTREE | |
| photos | 1 | photos_idx1 | 1 | userid | A | 5773 | NULL | NULL | YES | BTREE | |
| photos | 1 | photos_idx2 | 1 | dateuploaded | A | 57736 | NULL | NULL | YES | BTREE | |
| photos | 1 | photos_idx3 | 1 | camera | A | 17 | NULL | NULL | YES | BTREE | |
| photos | 1 | photos_idx4 | 1 | focallength | A | 308 | NULL | NULL | YES | BTREE | |
| photos | 1 | photos_idx5 | 1 | category | A | 96 | NULL | NULL | YES | BTREE | |
| photos | 1 | photos_idx6 | 1 | active | A | 1 | NULL | NULL | YES | BTREE | |
| photos | 1 | photo_idx7 | 1 | aperture | A | 354 | NULL | NULL | YES | BTREE | |
| photos | 1 | photos_idx8 | 1 | lenstype | A | 1 | NULL | NULL | YES | BTREE | |
| photos | 1 | photos_idx9 | 1 | film | A | 1 | NULL | NULL | YES | BTREE | |
| photos | 1 | photos_idx10 | 1 | originalfile | A | 57736 | NULL | NULL | YES | BTREE | |
+--------+------------+--------------+--------------+--------------+-----------+-------------+----------+--------+------+------------+---------+
有任何想法吗？谢谢！
解决方法:
此查询将返回(最多)20张不同的照片(“ by_photocomments”表上的ID值由升序标识的“ recent-cy”)的最“最近”的注释.返回的20张照片将是具有“最新”评论的照片,并应从最“最新”的评论开始排序. (注意：仅考虑bs_photocomments中具有active = 1的行,忽略bs_photocomments中的所有其他行,就好像它们甚至不在表中一样.)
SELECT p.photoid
, c.comment
, p.path
, p.smallfile
, p.bigfile
, c.userid
, c.dateposted
, u.username
, c.likephoto
JOIN (
SELECT n.photoid
, MAX(n.id) AS max_id
FROM bs_photocomments n
WHERE n.active = 1
GROUP BY n.photoid
ORDER BY max_id DESC
LIMIT 20
) m
JOIN bs_photocomments c
ON c.id = m.max_id
JOIN photos p
ON p.photoid = c.photoid
JOIN user u
ON u.userid = c.userid
ORDER BY c.id DESC
LIMIT 20
“技巧”是使用内联视图获取每个photoid的“最新”注释(内联视图的别名为m).一旦有了,只需从bs_photocomments中获取该行,以及从用户和照片中获取相关的行.
我相信原始查询中真正的性能“杀手”是联接谓词中的相关子查询.就对bs_photocomments表中的每一行执行该查询,甚至对photos表中的每一行运行该查询而言,这将是非常昂贵的.
为了获得最佳性能,您将需要“覆盖索引”,而不是每个单独列上的(无用的)索引.至少,您需要在bs_photocomments上具有以photoid作为前导索引的索引,然后是id.包括活动列将意味着可以在不访问任何数据页的情况下满足“使用索引”的内联视图查询.
... ON bs_photocomments (photoid, id, active)
使用该索引,MySQL应该能够相当有效地获取每个photoid的最大id.同样,有一个索引
... ON bs_photocomments (id, comment, userid, dateposted, likephoto)
这意味着针对bs_photocomments表(c)的外部查询也可以“使用索引”得到满足. (MySQL可能能够从索引满足ORDER BY,而无需进行排序操作.(您需要检查EXPLAIN输出,以查看“使用文件排序”是否出现在Extra列中.)
同样,这些覆盖索引可能会稍微提高查询的性能. (如果在不访问基础表中的页面的情况下通过索引满足查询,则EXPLAIN输出将显示“正在使用索引”.)
ON photos (photoid, path, smallfile, bigfile)
ON user (userid, username)
每个单独列上的大多数单独索引都是无用的；这些索引中的任何一个都不太可能被使用,因此它们所做的只是消耗资源.
标签：query-performance,mysql 来源： https://codeday.me/bug/20191031/1975495.html