加入和组多个表

Question

可以说，我有我的MSSQL 2016（后来Azure的SQL）

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

这种关系我们有：50米板，20万个主题，100万发的帖子和50K用户

的目标现在是一个板内

线程数

• 有ID和名称所有电路板
• 一板内一些职位
• 最新张贴板
• 用户ID和最新帖子

的名字在我的首秀

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 

它有一个令人难以置信的糟糕表现。

没有

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

它花费45ms，大约有6秒。

另一个节目是这样一个

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 

这是确定的，约172ms - 但没有最新帖子。

但我认为我在对冲的错误一面。并提示我如何达到我的目标？

Answer 1

好吧，这似乎是一个类似于论坛的项目，所以首先要记住的是：您的阅读方式比写在您的数据库上要多。

避免每次有人显示您的首页时应运行复杂的查询，这不是一个好主意，您只会结束一个slu DB的数据库。

这种问题在哪里可以通过奇迹触发。

在board增加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至母体ForumBoards.nb_threads
• ForumThreads.trgDeleteThread => -1至母体ForumBoards.nb_threads
• ForumPosts.trgAddPost => 1至母体ForumBoards.nb_posts，设置将当前post.id转换为last_post_id，将user.id转换为last_user_id并获取user.name以设置last_user_name
• ForumPosts.trgDeletePost => -1至父代ForumBoards.nb_posts并取回最后一个帖子以更新t他以前的数据

---

如果您不能使用触发器（如你在评论解释），这个查询应该做400毫秒下的伎俩：

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