加入和分组多个表

Question

假设我在我的 MSSQL 2016（后来的 Azure SQL）上有这种关系

[ForumBoards] 1-n [ForumThreads] 1-n [ForumPosts] n-1 [Users]

我们有：50 个版块、20 万个线程、100 万个帖子和 5 万个用户

现在的目标是

• 所有带有 ID 和名称的板
• 板内线程数
• 板内的帖子数
• 板内的最新帖子
• 最新帖子的用户 ID 和名称

我的第一个节目是

SELECT 
    Boards.Id AS BoardsId,
    Board.Name AS BoardsName,
    LP.*
        ThreadsCount = (SELECT Count(*) FROM ForumBoards AS SubB 
                        JOIN ForumThreads AS SubT ON SubB.Id = SubT.BoardId 
                        WHERE SubB.Id = Boards.Id AND SubT.BoardId = SubB.Id),  
        PostsCount = (SELECT Count(*) FROM ForumBoards AS SubB 
                        JOIN ForumThreads AS SubT ON SubB.Id = SubT.BoardId 
                        JOIN ForumPosts AS SubP ON SubT.Id = SubP.ThreadId 
                      WHERE SubB.Id = Boards.Id AND SubT.BoardId = SubB.Id AND SubP.ThreadId = SubT.Id)
FROM ForumBoards as Boards 

OUTER APPLY(
    SELECT 
    TOP 1   SubP.Id AS LatestPostId,
        SubP.PostedOn AS LatestPostPostedOn,
        SubP.ThreadId AS LatestPostThreadId,
        SubT.Topic AS LatestPostThreadTopic,
        SubU.Id AS LatestPostUserId,
        SubU.Username AS LatestPostUsername 
        FROM ForumBoards AS SubB 
             JOIN ForumThreads AS SubT ON SubB.Id = SubT.BoardId 
             JOIN ForumPosts AS SubP ON SubT.Id = SubP.ThreadId 
             JOIN Users AS SubU ON SubP.UserId = SubU.Id 
        WHERE SubB.Id = Boards.Id AND SubT.BoardId = SubB.Id AND SubP.ThreadId = SubT.Id AND SubU.Id = SubP.UserId 
        ORDER BY SubP.PostedOn DESC) AS LP

性能非常差。

没有

WHERE SubB.Id = Boards.Id AND SubT.BoardId = SubB.Id AND SubP.ThreadId = SubT.Id AND SubU.Id = SubP.UserId 

花费 45 毫秒，大约 6 秒。

另一个节目是这个

SELECT  
    B.Id,
    B.Name as BoardName,
    Count(*) as ThreadsCount,
    (SELECT Count(*)
    FROM
        ForumBoards Boards, ForumThreads Threads, ForumPosts Posts
        WHERE Boards.Id = Threads.BoardId AND Threads.Id = Posts.ThreadId AND Boards.Id = B.Id) AS PostsCount

FROM    ForumBoards B, ForumThreads T

WHERE B.Id = T.BoardId

GROUP   BY B.Id, B.Name

没关系，大约 172 毫秒 - 但没有最新的帖子。

但我认为我在对冲的错误一边。并提示我如何实现目标？

Answer 1

好的，这似乎是一个类似论坛的项目，所以首先要记住：你的阅读量要比在数据库上写的多。

避免每次有人显示您的首页时都应运行的复杂查询，这不是一个好主意，您只会以缓慢的数据库结束。

这种问题是触发器可以神奇的地方。

在 board 上添加 5 个新列：

• nb_threads
• nb_posts
• last_post_id
• last_user_id
• last_user_name

并添加以下触发器：

• ForumThreads.trgAddThread => +1 给家长ForumBoards.nb_threads
• ForumThreads.trgDeleteThread => -1 给父 ForumBoards.nb_threads
• ForumPosts.trgAddPost => +1 到父 ForumBoards.nb_posts，将当前 post.id 设置为 last_post_id，将 user.id 设置为 last_user_id 并获取 user.name 以设置 last_user_name
• ForumPosts.trgDeletePost => -1 到父 ForumBoards.nb_posts 并获取最后一个帖子以更新以前的数据

---

如果你不能使用触发器（正如你在评论中解释的那样），这个查询应该在 400 毫秒内完成：

SELECT 
    Boards.Id AS BoardsId,
    Boards.Name AS BoardsName,
    coalesce(nbThread.ThreadsCount, 0) ThreadsCount, 
    coalesce(LP.nbPost, 0) nbPost,
    LP.*,
FROM ForumBoards AS Boards
LEFT JOIN (
    SELECT BoardId, Count(*) ThreadsCount
    FROM ForumThreads
    GROUP BY ForumThreads.BoardId
) AS nbThread
    ON nbThread.BoardId = Boards.Id
LEFT JOIN (
    SELECT   
        t.BoardId,
        t.nbPost,
        ForumPosts.Id AS LatestPostId,
        ForumPosts.PostedOn AS LatestPostPostedOn,
        ForumPosts.ThreadId AS LatestPostThreadId,
        ForumThreads.Topic AS LatestPostThreadTopic,
        Users.Id AS LatestPostUserId,
        Users.Username AS LatestPostUsername 
        FROM (
            select 
                ForumThreads.BoardId, 
                MAX(ForumPosts.Id) Id, 
                Count(*) nbPost
            from ForumPosts
            JOIN ForumThreads
                ON ForumThreads.Id = ForumPosts.ThreadId
            GROUP BY ForumThreads.BoardId
        ) AS t 
        INNER JOIN ForumPosts  
            ON t.Id = ForumPosts.Id 
        INNER JOIN ForumThreads
            ON ForumThreads.Id = ForumPosts.ThreadId
        INNER JOIN Users
            ON Users.Id = ForumPosts.UserId        
) AS LP
    ON LP.BoardId = Boards.Id