Question

我的django应用中的模型有发布/回复关系，我试图在最新回复时对帖子进行排序，或者如果没有回复，则自己的时间戳。这就是我现在所拥有的：

threads = ConversationThread.objects.extra(select={'sort_date':
                                    """select case when (select count(*) from conversation_conversationpost 
                                    where conversation_conversationpost.thread_id = conversation_conversationthread.id) > 0 
                                    then (select max(conversation_conversationpost.post_date) 
                                    from conversation_conversationpost where conversation_conversationpost.thread_id = conversation_conversationthread.id) 
                                    else conversation_conversationthread.post_date end"""}).order_by('-sort_date')

虽然它有效但我预感到这不是最简洁或最有效的方法。什么是更好的方式？

Answer 1

SELECT  *,
        (
        SELECT  COALESCE(MAX(cp.post_date), ct.post_date)
        FROM    conversation_conversationpost cp
        WHERE   cp.thread_id = ct.id
        ) AS sort_date
FROM    conversation_conversationthread ct
ORDER BY
        sort_date DESC

Answer 2

众所周知，{p> Correlated subqueries很慢 LEFT JOIN可能会快得多（取决于数据分布）：

SELECT  t.*, COALESCE(p.max_post_date, t.post_date) AS sort_date
FROM    conversation_conversationthread t
LEFT    JOIN (
    SELECT thread_id, MAX(post_date) AS max_post_date
    FROM   conversation_conversationpost
    GROUP  BY thread_id
    ) p ON p.thread_id = t.id
ORDER   BY sort_date DESC;

按最新孩子的时间戳或自己的时间戳对帖子进行排序

2 个答案: