关于从表中选择最后n条记录的性能

Question

我正在查询表以选择最后n条记录，但保留了顺序。对于这一点，我使用下面的查询这是我从Select Top N Records Ordered by X, But Have Results in Reverse Order有：

WITH Temp 
      AS (SELECT TOP 10 
         [TestID] , 
         UserID , 
         DateSent 
       FROM  [Test] 
       WHERE UserID = @UserID 
       ORDER BY DateSent DESC 
      ) 
    SELECT * 
    FROM Temp 
    ORDER BY DateSent 

即我选择最后N条记录，同时保留排序顺序。下面是空的脚本创建上述表：

GO 
CREATE TABLE [dbo].[Test] 
    (
     [TestID] [int] IDENTITY(1, 1) 
        NOT NULL , 
     [UserID] [int] NOT NULL , 
     [DateSent] [datetime] NOT NULL , 
     CONSTRAINT [PK_TestID] PRIMARY KEY CLUSTERED ([TestID] ASC) 
     WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, 
       IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, 
       ALLOW_PAGE_LOCKS = ON) ON [PRIMARY] 
    ) 
ON [PRIMARY] 
GO 

CREATE NONCLUSTERED INDEX [IX_Test_UserID_DateSent] ON [dbo].[Test] 
(
    [UserID] ASC, 
    DateSent DESC 
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, SORT_IN_TEMPDB = OFF, IGNORE_DUP_KEY = OFF, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY] 
GO 


GO 

INSERT INTO [Test] 
SELECT TOP 100000 ABS(CAST(NEWID() AS BINARY(6)) %10), 
DATEADD(day, DATEDIFF(day, 0, GETDATE()) - 1 - FLOOR(RAND(CAST(NEWID() AS binary(4))) * 365.25 * 90), 0) 
FROM master..spt_values 
GO 

我在上文中创建的表，就可以申请指数，并在其中插入一些虚拟的数据。我执行此查询来获取记录：

DECLARE @UserID INT 
SET @UserID = 1 ; 
WITH Temp 
      AS (SELECT TOP 10 
         [TestID] , 
         UserID , 
         DateSent 
       FROM  [Test] 
       WHERE UserID = @UserID 
       ORDER BY DateSent DESC 
      ) 
    SELECT * 
    FROM Temp 
    ORDER BY DateSent 

下面是运行上面的查询后的执行计划：

正如你可以看到指数以下，但对于内部查询和您可以在索引计划中看到77％的执行是由排序过程执行的。 我该如何避免这种情况？我应该在这里应用什么指数来克服这种情况。

Answer 1

消耗77％执行计划的最左侧排序仅适用于您的TOP 10记录。您可以通过删除最后ORDER BY验证这一点：

DECLARE @UserID INT 
SET @UserID = 1 ; 
WITH Temp 
      AS (SELECT TOP 10 
         [TestID] , 
         UserID , 
         DateSent 
       FROM  [Test] 
       WHERE UserID = @UserID 
       ORDER BY DateSent DESC 
      ) 
    SELECT * 
    FROM Temp 
    --ORDER BY DateSent 

应当提及的是，在一个计划中的所有运营商必须为100％。如果您最便宜的操作（即仅分类10条记录）消耗大部分执行时间，那么我会说您状态良好。