加入和組多個表

Question

可以說，我有我的MSSQL 2016（後來Azure的SQL）

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

這種關係我們有：50米板，20萬個主題，100萬發的帖子和50K用戶

的目標現在是一個板內

線程數

• 有ID和名稱所有電路板
• 一板內一些職位
• 最新張貼板
• 用戶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 

它有一個令人難以置信的糟糕表現。

沒有

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

它花費45ms，大約有6秒。

另一個節目是這樣一個

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 

這是確定的，約172ms - 但沒有最新帖子。

但我認爲我在對衝的錯誤一面。並提示我如何達到我的目標？

Answer 1

好吧，這似乎是一個類似於論壇的項目，所以首先要記住的是：您的閱讀方式比寫在您的數據庫上要多。

避免每次有人顯示您的首頁時應運行復雜的查詢，這不是一個好主意，您只會結束一個slu DB的數據庫。

這種問題在哪裏可以通過奇蹟觸發。

在board增加5個新列：

• nb_threads
• nb_posts
• last_post_id
• last_user_id
• last_user_name

並添加以下觸發器：

• ForumThreads.trgAddThread => 1至母體ForumBoards.nb_threads
• ForumThreads.trgDeleteThread => -1至母體ForumBoards.nb_threads
• ForumPosts.trgAddPost => 1至母體ForumBoards.nb_posts，設置將當前post.id轉換爲last_post_id，將user.id轉換爲last_user_id並獲取user.name以設置last_user_name
• ForumPosts.trgDeletePost => -1至父代ForumBoards.nb_posts並取回最後一個帖子以更新t他以前的數據

---

如果您不能使用觸發器（如你在評論解釋），這個查詢應該做400毫秒下的伎倆：

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