当查询有 order by 子句时运行极其缓慢的复杂 sql

Question

我有以下需要使用的复杂查询。当我运行它时，它需要 30 到 40 秒。但是如果我删除 order by 子句，返回结果需要 0.0317 秒，这与 30 秒或 40 秒相比非常快。

select DISTINCT media.*
        , username 
from album as album
     , album_permission as permission
      , user as user, media as media
where ((media.album_id = album.album_id 
        and album.private = 'yes' 
        and album.album_id = permission.album_id 
        and (permission.email = '' or permission.user_id = '') ) 
or (media.album_id = album.album_id 
      and album.private = 'no' ) 
or media.album_id = '0' ) 
and media.user_id = user.user_id
and media.media_type = 'video'
order by media.id DESC 
LIMIT 0,20  

order by 上的 id 是主键，它也被索引。所以我不知道是什么问题。

我也有专辑和专辑权限表，只是为了检查媒体是公共的还是私人的，如果是私人的，那么检查用户是否有权限。我在想这可能是导致问题的原因。如果我在子查询中这样做会更好吗？如果这是解决方案，也有人可以帮我写那个子查询吗？如果你忍不住写，至少告诉我。这个问题我真的要疯了..

可能的解决方案

是的，我认为子查询是最好的解决方案，因为以下查询运行时间为 0.0022 秒。但我不确定专辑验证是否准确，请检查。

select media.*, username 
from media as media
      , user as user
where  media.user_id = user.user_id 
and media.media_type = 'video'
and media.id in 
    (select media2.id
    from     media as media2
            , album as album
            , album_permission as permission
     where ((media2.album_id = album.album_id  
             and album.private = 'yes'
             and album.album_id = permission.album_id 
            and (permission.email = '' 
                 or permission.user_id = ''))
             or (media.album_id = album.album_id 
                   and album.private = 'no' ) 
              or media.album_id = '0' ) 
     and media.album_id = media2.album_id )            
order by media.id DESC
LIMIT 0,20   

Answer 1

使用 EXPLAIN 来确定为什么没有 ORDER BY 子句会更快：

http://dev.mysql.com/doc/refman/5.1/en/using-explain.html

我也建议使用 ANSI 连接语法重写查询。这可能有助于性能。像这样的：

select DISTINCT media.*
        , username 
from album as album
  inner join media as media on media.album_id = album.album_id
  inner join user as user on media.user_id = user.user_id
  left outer join album_permission as permission on album.album_id = permission.album_id
where ((album.private = 'yes' 
        and (permission.email = '' or permission.user_id = '') ) 
or album.private = 'no' 
or media.album_id = '0' ) 
and media.media_type = 'video'
order by media.id DESC 
LIMIT 0,20  

您还可以将查询分成 2 或 3 个不同的查询并将它们联合在一起以避免外连接。