Question

我有一个非常大的网络论坛应用程序（自2001年以来大约有2000万个帖子）从SQL Server 2012数据库运行。数据文件大小约为40GB。

我在相应字段的表格中添加了索引，但是此查询（显示每个论坛中帖子的日期范围）大约需要40分钟才能运行：

SELECT
    T2.ForumId,
    Forums.Title,
    T2.ForumThreads,
    T2.ForumPosts,
    T2.ForumStart,
    T2.ForumStop

FROM
    Forums
    INNER JOIN (

    SELECT
        Min(ThreadStart) As ForumStart,
        Max(ThreadStop) As ForumStop,
        Count(*) As ForumThreads,
        Sum(ThreadPosts) As ForumPosts,
        Threads.ForumId
    FROM
        Threads
        INNER JOIN (

            SELECT
                Min(Posts.DateTime) As ThreadStart,
                Max(Posts.DateTime) As ThreadStop,
                Count(*) As ThreadPosts,
                Posts.ThreadId
            FROM
                Posts
            GROUP BY
                Posts.ThreadId

        ) As P2 ON Threads.ThreadId = P2.ThreadId

    GROUP BY
        Threads.ForumId

) AS T2 ON T2.ForumId = Forums.ForumId

我怎样才能加快速度？

更新：

这是从右到左的预估执行计划：

[Path 1]

Clustered Index Scan (Clustered) [Posts].[PK_Posts], Cost: 98%
Hash Match (Partial Aggregate), Cost: 2%
Parallelism (Repartition Streams), Cost: 0%
Hash Match (Aggregate), Cost 0%
Compute Scalar, Cost: 0%
Bitmap (Bitmap Create), Cost: 0%

[Path 2]

Index Scan (NonClustered) [Threads].[IX_ForumId], Cost: 0%
Parallelism (Repartition Streams), Cost: 0%

[Path 1 and 2 converge into Path 3]

Hash Match (Inner Join), Cost: 0%
Hash Match (Partial Agregate), Cost: 0%
Parallelism (Repartition Streams), Cost: 0%
Sort, Cost: 0%
Stream Aggregate (Aggregate), Cost: 0%
Compute Scalar, Cost: 0%

[Path 4]
Clustered Index Seek (Clustered) [Forums].[PK_Forums], Cost: 0%

[Path 3 and 4 converge into Path 5]

Nested Loops (Inner Join), Cost: 0%
Paralleism (Gather Streams), Cost: 0%
SELECT, Cost: 0%

Answer 1

您是否尝试将这2个派生表放在#temp表中？ SQL Server将从它们获取统计信息（单列），您也可以在它们上面建立索引。

此外，乍一看，索引视图可能有所帮助，因为您有很多聚合。

Answer 2

这样的事情怎么样？无论如何，你明白了......

SELECT f.ForumID,
f.Title,
MIN(p.[DateTime]) as ForumStart,
MAX(p.[DateTime]) as ForumStop,
COUNT(DISTINCT f.ForumID) as ForumPosts,
COUNT(DISTINCT t.ThreadID) as ForumThreads
FROM Forums f
INNER JOIN Threads t
ON f.ForumID = t.ForumID
INNER JOIN Posts p
ON p.ThreadID = p.ThreadID
GROUP BY f.ForumID, f.Title

Answer 3

当您执行SELECT FROM时索引可能有效，但子查询的结果未编入索引。加入他们可能会扼杀他们的表现。

正如Buckley建议的那样，我会尝试将中间结果存储在#temp表中，并在进行最终查询之前添加索引。

但外部SELECT不包含特定于线程的信息。看起来查询只是按论坛选择最小/最大日期。如果是这样，您可以获得按论坛分组的最小/最大/计数帖子。

Answer 4

你真的需要聚合两次吗？这个查询会给你相同的结果吗？

SELECT 
T2.ForumId, 
Forums.Title, 
T2.ForumThreads, 
T2.ForumPosts, 
T2.ForumStart, 
T2.ForumStop  
FROM 
    Forums 
INNER JOIN (  
    SELECT
         Min(ThreadStart) As ForumStart,
         Max(ThreadStop) As ForumStop,     
         Count(*) As ForumThreads,     
         Sum(ThreadPosts) As ForumPosts,     
         Threads.ForumId 
    FROM     
        Threads     
    INNER JOIN (          
                SELECT             
                    Posts.DateTime As ThreadStart,             
                    Posts.DateTime As ThreadStop,             
                    Count(*) As ThreadPosts,             
                    Posts.ThreadId         
                FROM             
                    Posts         
                 ) As P2 ON Threads.ThreadId = P2.ThreadId  
    GROUP BY     
        Threads.ForumId  
    ) AS T2 ON T2.ForumId = Forums.ForumId

Answer 5

如果通过将ForumId添加到Posts表来进行非规范化，则可以直接从Posts表中查询所有统计信息。使用正确的索引，这可能会表现得相当不错。当然，这需要对代码进行少量更改，以便在插入Posts表时包含ForumId ...

Answer 6

我在数据库中添加了一些索引，并且大大加快了速度。执行时间现在约为20秒（!!）。我承认很多添加的索引都是猜测（或者只是随机添加）。

SQL Server - 嵌套查询需要40分钟才能运行

6 个答案: