如何优化嵌套查询

Question

我有很多这样的查询：

     select  thread_id as topic_id,title as topic            
  ,            
    isnull((select count(*) from tblmessages b where thread_id=a.thread_id and is_approved='Y' and sort_level>1            
     group by b.thread_id            
     ),0) as replies,            
    isnull((select count(*) from tblmessages b where thread_id=a.thread_id and isnull(new_post,'Y')='Y' and sort_level>1            
     group by b.thread_id            
     ),0) as NewPost,            
    isnull((select count(*) from tblmessages b where thread_id=a.thread_id and isnull(is_approved,'N')='N' and sort_level>1            
     group by b.thread_id            
     ),0) as NotClear,            

    sort_level,sort_index,  from tblMessages a            
    where   sort_level=1 and category=@category 
    order by topic_id desc

请告诉我如何优化和更好地编写此类查询。因为我有记录 5,00,000 的表。所以这需要很多时间，而且有时会超时。

谢谢

Answer 1

您应该将各种子查询分组到一个具有不同计数的单个子查询中，并使用连接将数据放在一起

子查询应该是：

select thread_id
     count(when isnull(is_approved,'N')='N' then 1 end) as replies,
     count(when isnull(new_post,'Y')='Y' then 1 end) as NewPost,
     count(when isnull(is_approved,'N')='N' then 1 end) as NotClear
     from tblmessages 
     where sort_level>1  
     group by thread_id

虽然最终查询如下

select  thread_id as topic_id,title as topic,
   sort_level,sort_index , B.replies, B.NewPost, B.NotClear
   from tblMessages a            
   join
     (select thread_id
     count(when isnull(is_approved,'N')='N' then 1 end) as replies,
     count(when isnull(new_post,'Y')='Y' then 1 end) as NewPost,
     count(when isnull(is_approved,'N')='N' then 1 end) as NotClear
     from tblmessages 
     where sort_level>1  
     group by thread_id) as B
     on a.thread_id = B.thread_id
    where   sort_level=1 and category=@category 
    order by topic_id desc

Answer 2

你可以尝试去规范化一点：

1. 创建replies、NewPost 和NotClear 字段
2. 编写一个更新这些字段的例程，cron 它（周期取决于 3。）
3. 重写影响这些字段的大多数/所有查询以更新它们。如果你重写所有，运行 2. 一天几次。否则，根据您需要的数据完整性，每小时执行几次。

这显然有助于您的查询。但是，它需要更多的维护，因为几乎从未使用过的任何小查询都会破坏一致性（想想一些针对 BB 的调节工具，例如拆分主题......）

Answer 3

SELECT a.* 
FROM 
    (SELECT   
        thread_id AS topic_id,
        title AS topic ,
        SUM(CASE WHEN is_approved='Y' AND sort_level > 1 THEN 1 ELSE 0 END) as replies, 
        SUM(CASE WHEN isnull(new_post,'Y')='Y' AND sort_level > 1 THEN 1 ELSE 0) END as NewPost, 
        SUM(CASE WHEN isnull(is_approved,'N')='N' AND sort_level > 1 THEN 1 ELSE 0 END) as NotClear, 
        sort_level ,
        sort_index,  
        category ,
        topic_id
    FROM 
        tblMessages 
    ) a
WHERE 
    a.sort_level=1 AND a.category=@category 
ORDER BY 
        a.topic_id DESC

我无法对此进行测试，所以可能存在一些语法错误，但你明白了吗？