当查询按子句排序时运行速度非常慢的复杂sql

Question

我有以下复杂的查询，我需要使用它。当我运行它时，需要30到40秒。但是，如果我一条一条地删除订单，需要0.0317秒，返回的结果，这是真快，比较30秒或40

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 

上order by的ID是太索引的主键。所以我不知道是什么问题。

我也有相册和相册权限表，只是为了检查媒体是公共还是私人，如果是私人的，那么检查用户是否有权限。我想也许这是造成这个问题。如果我在子查询中这样做了，会更好吗？如果这是解决方案，也可以有人帮我写这个子查询吗？如果你不能写出来，至少告诉我。我真的要疯了这个问题..

SOLUTION MAYBE

是的，我觉得子查询将是这个最佳的解决方案，因为下面的查询在0.0022秒运行。但我不确定专辑的验证是否准确，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来确定它为什么没有ORDER更快BY子句：

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

我会建议用ANSI连接语法太重写查询。这可能有助于表现。例如：

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 

您也可以将查询分为2或3个不同的查询，并将它们联合在一起以避免外部联接。