sql-server - SQL 服务器 2008 : Very slow query performance when dealing with specific date boundary

Question

没错，这很奇怪。

我们有一个将数据返回给 silverlight 客户端的网络服务。使用 Entity Framework 针对 SQL Server 2008 数据库生成查询。大多数这些查询都是基于日期范围的——例如，在这个日期和那个日期之间提取结果。此外， View 用于使事情变得更容易一些。

我们注意到，当发出一组在特定日期或之后开始的特定查询时，执行速度会非常慢。这个日期是每年的 11 月 5 日。如果我们的开始日期早一天，执行将是即时的。所以，第 5 到第 6，慢。第 4 - 6 - 快。

这是 EF 生成的 SQL:

这个查询会非常慢(30 秒)

SELECT 
1 AS [C1], 
[GroupBy1].[K1] AS [Name], 
[GroupBy1].[A1] AS [C2]
FROM ( SELECT 
[Extent1].[Name] AS [K1], 
SUM([Extent1].[Value]) AS [A1]
FROM (SELECT 
  [view_answers].[Value] AS [Value], 
  [view_answers].[Comment] AS [Comment], 
  [view_answers].[NewStockist] AS [NewStockist], 
  [view_answers].[NewDistPoint] AS [NewDistPoint], 
  [view_answers].[VoucherUsed] AS [VoucherUsed], 
  [view_answers].[CashTotal] AS [CashTotal], 
  [view_answers].[AnswerType] AS [AnswerType], 
  [view_answers].[StartTime] AS [StartTime], 
  [view_answers].[ActualEndTime] AS [ActualEndTime], 
  [view_answers].[Complete] AS [Complete], 
  [view_answers].[UserID] AS [UserID], 
  [view_answers].[UserName] AS [UserName], 
  [view_answers].[QuestionType] AS [QuestionType], 
  [view_answers].[ProductSKU] AS [ProductSKU], 
  [view_answers].[BrandID] AS [BrandID], 
  [view_answers].[TeamID] AS [TeamID], 
  [view_answers].[Name] AS [Name], 
  [view_answers].[Stage] AS [Stage], 
  [view_answers].[Question] AS [Question]
  FROM [dbo].[view_answers] AS [view_answers]) AS [Extent1]
  WHERE 
     ([Extent1].[UserID] = '16E3692F-806E-40A0-BB99-ABBBCC13060D') 
     AND (N'Distribution Points' = [Extent1].[QuestionType]) 
     AND ([Extent1].[StartTime] >= '11/05/2009 00:00:00') 
     AND ([Extent1].[StartTime] <= '11/08/2009 00:00:00') 
     AND (1 = [Extent1].[Complete]) 
     AND (2 = [Extent1].[BrandID]) 
     AND (N'Distribution Points' = [Extent1].[QuestionType])
  GROUP BY 
     [Extent1].[Name])  
  AS [GroupBy1]

这个会快得多

    SELECT 
    1 AS [C1], 
    [GroupBy1].[K1] AS [Name], 
    [GroupBy1].[A1] AS [C2]
    FROM ( SELECT 
    [Extent1].[Name] AS [K1], 
    SUM([Extent1].[Value]) AS [A1]
    FROM (SELECT 
      [view_answers].[Value] AS [Value], 
      [view_answers].[Comment] AS [Comment], 
      [view_answers].[NewStockist] AS [NewStockist], 
      [view_answers].[NewDistPoint] AS [NewDistPoint], 
      [view_answers].[VoucherUsed] AS [VoucherUsed], 
      [view_answers].[CashTotal] AS [CashTotal], 
      [view_answers].[AnswerType] AS [AnswerType], 
      [view_answers].[StartTime] AS [StartTime], 
      [view_answers].[ActualEndTime] AS [ActualEndTime], 
      [view_answers].[Complete] AS [Complete], 
      [view_answers].[UserID] AS [UserID], 
      [view_answers].[UserName] AS [UserName], 
      [view_answers].[QuestionType] AS [QuestionType], 
      [view_answers].[ProductSKU] AS [ProductSKU], 
      [view_answers].[BrandID] AS [BrandID], 
      [view_answers].[TeamID] AS [TeamID], 
      [view_answers].[Name] AS [Name], 
      [view_answers].[Stage] AS [Stage], 
      [view_answers].[Question] AS [Question]
      FROM [dbo].[view_answers] AS [view_answers]) AS [Extent1]
    WHERE 
       ([Extent1].[UserID] = '16E3692F-806E-40A0-BB99-ABBBCC13060D') 
       AND (N'Distribution Points' = [Extent1].[QuestionType]) 
       AND ([Extent1].[StartTime] >= '11/04/2009 00:00:00') 
       AND ([Extent1].[StartTime] <= '11/08/2009 00:00:00') 
       AND (1 = [Extent1].[Complete]) 
       AND (2 = [Extent1].[BrandID]) 
       AND (N'Distribution Points' = [Extent1].[QuestionType])
    GROUP BY 
       [Extent1].[Name])  
    AS [GroupBy1]

如果我们将开始日期设置为去年 11 月 5 日，执行会很慢，去年 11 月 4 日，然后又很快。查看数据库中的数据，5号左右没有异常。此外，开始日期在 5 号之后的查询似乎运行缓慢。

我被难住了!

(数据库是远程托管的，所以我不能直接访问它)

更新

谢谢大家的回复。首先，我可能应该明确表示我对 sql server 的了解很肤浅。我构建了不同质量等级的数据库，然后使用 Linq To SQL 或 EF 之类的工具来处理它们。所以我在这里感觉有点力不从心。

关于连接——我正在查询的 View 确实包含来自大约 6-7 个表的数据。下次工作时，我将尝试获取一些统计数据，并在此处添加更多信息。我不太了解执行计划，或者它们是我可以通过 SQL Server Management Studio 看到的东西

更新 来自慢查询的统计 (3 行受影响)

Table 'tblProducts'. Scan count 0, logical reads 22, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'tblQuestionTypes'. Scan count 0, logical reads 1496, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'tblQuestions'. Scan count 0, logical reads 1496, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'tblSessions'. Scan count 0, logical reads 28551, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'tblAnswers'. Scan count 1, logical reads 1976256, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'tblCalls'. Scan count 1, logical reads 439, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'tblUsers'. Scan count 0, logical reads 2, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

快速查询统计

Table 'Worktable'. Scan count 0, logical reads 0, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'tblAnswers'. Scan count 1, logical reads 7008, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'tblProducts'. Scan count 0, logical reads 22, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'tblQuestions'. Scan count 1, logical reads 4, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'tblQuestionTypes'. Scan count 1, logical reads 2, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'tblCalls'. Scan count 1, logical reads 439, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'tblSessions'. Scan count 1, logical reads 47, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'tblUsers'. Scan count 0, logical reads 2, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

更新 查询执行计划(对我来说是新事物)建议我向其中一个表中的行添加一个新索引 - answers 表中的 questionid。建议的sql如下:

USE [brandfourcoke]
GO
CREATE NONCLUSTERED INDEX [<Name of Missing Index, sysname,>]
ON [dbo].[tblAnswers] ([QuestionID])
INCLUDE ([CallID],[Value])
GO

查询成本的估计改进为 93%。 我应该继续这样做吗？该数据库处于实时环境中，并使用 sql server 2008 的更改跟踪和同步框架。因此，我一直担心数据库更改会影响跟踪数据并破坏我的客户。添加索引重置会更改跟踪数据吗？ 谢谢大家的帮助。我觉得自己在这里完全是个初学者。

Answer 1

使用 SET sTATISTICS IO ON 运行慢速和快速查询，看看两者之间的逻辑读取/物理读取量是否存在显着差异。

数据很可能存在严重偏差。例如，快速计划执行由 10 行结果驱动的嵌套循环(导致 10 次嵌套查找)，而慢速计划突然看到 10000 行，而前一个计划看到 10 行，导致 10000 次查找。尽管您的查询没有连接，但引擎可能会使用各种访问索引并将索引与集群索引连接起来。实际的执行计划总是会准确地揭示正在发生的事情。