Question

我知道之前已经提出过这样的问题，但没有一个能让我找到解决方案。

这是我的问题：

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AND的一点解释：

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，我只想显示最后一条。

ORDER BY c.id是杀手。只有180k评论和100k张照片。如果我删除了ORDER BY，则查询立即返回，但显然不符合我想要的结果。

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      |         |
+--------+------------+--------------+--------------+--------------+-----------+-------------+----------+--------+------+------------+---------+

有什么想法吗？谢谢！

Answer 1

此查询将返回（最多）20张不同照片的最近“最近”评论（在bs_photocomments表中通过升序ID值识别“recent-cy”）。返回的20张照片将是那些“最近”评论的照片，并且可以从最“最近”的评论开始订购。（注意：只考虑具有active=1的bs_photocomments中的行，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（内联视图别名为m）。一旦我们有了这个，只需要从bs_photocomments获取行，以及来自用户和照片的相关行。

我认为原始查询中真正的性能“杀手”是连接谓词中的相关子查询。就执行bs_photocomments表中每一行的查询，甚至为照片表中的每一行运行它而言，这将非常昂贵。

为获得最佳性能，您需要在每个单独的列上“覆盖索引”而不是（无用）索引。至少，您需要bs_photocomments的索引，其中photoid为主要索引，后跟id。包括active列将意味着内联视图查询可以满足“使用索引”，而无需访问任何数据页。

... ON bs_photocomments (photoid, id, active)

使用该索引，MySQL应该能够相当有效地获得每个photoid的最大id。同样，有一个索引

... ON bs_photocomments (id, comment, userid, dateposted, likephoto)

意味着针对bs_photocomments表（c）的外部查询也可以满足“使用索引”。（MySQL可能能够满足索引中的ORDER BY，而无需进行排序操作。（您需要检查EXPLAIN输出，以查看Extra列中是否显示“Using filesort”。）

此外，这些覆盖索引可能会略微提高查询的性能。（如果从索引中满足查询而不访问基础表中的页面，则EXPLAIN输出将显示“使用索引”。）

  ON photos (photoid, path, smallfile, bigfile)

  ON user (userid, username)

每个列上的大多数单独索引都是无用的;

。这些索引不太可能被使用，所以他们所做的就是消耗资源。

使用Order的Mysql查询太慢了

1 个答案: