1

I had executed the following procedure in SSMS: 

CREATE Procedure [dbo].[MyTableLoadByPK]  
(  
    @MyTableID int  
)  
AS  
BEGIN  
    SET NOCOUNT ON  
    SELECT  
    *  
    FROM [MyTable]  
    WHERE  
    del = 0 AND MyTableID = @MyTableID 
END  

exec MyTableLoadByPK @MyTableID=1001

MyTable is having 40000 records and there is a clustered index on MyTableID.

The STATISTICS IO were same every time I called the above procedure logical reads 2, physical reads 2:

Table 'MyTable'. Scan count 0, logical reads 2, physical reads 2, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

But when I checked the trace in Profiler for the same procedure, I found a huge difference in reads and even difference in difference. One time 248 and three times 228.

Following are reads from the trace:

TextData                                    CPU  Reads   Writes    Duration

exec MyTableLoadByPK @MyTableID=1001        0    228     0         223
exec MyTableLoadByPK @MyTableID=1001        0    228     0         230
exec MyTableLoadByPK @MyTableID=1001        0    248     0         751
exec MyTableLoadByPK @MyTableID=1001        0    228     0         360

Note: I had executed DBCC DROPCLEANBUFFERS; DBCC FREEPROCCACHE; before each four executions.

The execution plan is same in both SSMS and Profiler. There is only one Clustered Index Seek. Cost, rows, executions, size everything is same in both, then why there is so much difference in reads.

Edit:

MyTable create statement is as following:

CREATE TABLE [dbo].[MyTable](
    [MyTableID] [int] IDENTITY(1,1) NOT NULL,
    [na] [nvarchar](255) NOT NULL,
    [conID] [int] NOT NULL,
    [coID] [int] NOT NULL,
    [sID] [int] NOT NULL,
    [coID_O] [int] NOT NULL,
    [sID_O] [int] NOT NULL,
    [ufmts] [bit] NOT NULL,
    [Lte] [float] NOT NULL,
    [Late] [float] NOT NULL,
    [tz] [nvarchar](20) NOT NULL,
    [dm] [int] NOT NULL,
    [ca] [nvarchar](20) NOT NULL,
    [Tt] [nvarchar](50) NOT NULL,
    [Ct] [nvarchar](2048) NOT NULL,
    [pub] [bit] NOT NULL,
    [do] [int] NOT NULL,
    [cuID] [int] NOT NULL,
    [ia] [nvarchar](50) NOT NULL,
    [con] [datetime] NOT NULL,
    [uon] [datetime] NOT NULL,
    [upc] [int] NOT NULL,
    [del] [bit] NOT NULL,
 CONSTRAINT [PK_MyTable] PRIMARY KEY CLUSTERED 
(
    [MyTableID] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON, FILLFACTOR = 80) ON [PRIMARY]
) ON [PRIMARY]

GO

ALTER TABLE [dbo].[MyTable]  WITH CHECK ADD  CONSTRAINT [FK_MyTable_con] FOREIGN KEY([conID])
REFERENCES [dbo].[MYcon] ([conID])
GO

ALTER TABLE [dbo].[MyTable] CHECK CONSTRAINT [FK_MyTable_Mycon]
GO

ALTER TABLE [dbo].[MyTable]  WITH CHECK ADD  CONSTRAINT [FK_MyTable_Myco] FOREIGN KEY([coID])
REFERENCES [dbo].[Myco] ([coID])
GO

ALTER TABLE [dbo].[MyTable] CHECK CONSTRAINT [FK_MyTable_Myco]
GO

ALTER TABLE [dbo].[MyTable]  WITH CHECK ADD  CONSTRAINT [FK_MyTable_Mys] FOREIGN KEY([sID])
REFERENCES [dbo].[Mys] ([sID])
GO

ALTER TABLE [dbo].[MyTable] CHECK CONSTRAINT [FK_MyTable_Mys]
GO

ALTER TABLE [dbo].[MyTable] ADD  CONSTRAINT [DF_MyTable_upc]  DEFAULT ((0)) FOR [upc]
GO
4

1 回答 1

5

您不应该期望在这两种情况下（SET STATISTICS IO ON和SQL Profiler:Batch Completed）看到相同的逻辑读取值，因为它们测量不同的东西：

  • SET STATISTICS IO ON将只返回logical reads语句[s]，
  • SQL Profiler:Batch Completed column Reads将返回整个批处理的逻辑读取，包括此处由编译生成的逻辑读取（您的批处理具有强制编译的 DBCC FREEPROCCACHE 语句，这意味着逻辑读取，因为 SQL Server 必须读取元数据信息）。此外，如果您Query > Actual execution plan在 SQL Server Management Studio 中激活，您将看到Reads列增加）。

例如，当我在 SSMS 中运行这两个批次时

-- Without Query > Actual Execution Plan
DBCC DROPCLEANBUFFERS; 
DBCC FREEPROCCACHE;
exec MyTableLoadByPK @MyTableID=1001
GO
-- With Query > Actual Execution Plan
DBCC DROPCLEANBUFFERS; 
DBCC FREEPROCCACHE;
exec MyTableLoadByPK @MyTableID=1001
GO

的输出SET STATISTICS IO ON是0 个逻辑读取

DBCC execution completed. If DBCC printed error messages, contact your system administrator.
DBCC execution completed. If DBCC printed error messages, contact your system administrator.
Table 'MyTable'. 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.
DBCC execution completed. If DBCC printed error messages, contact your system administrator.
DBCC execution completed. If DBCC printed error messages, contact your system administrator.
Table 'MyTable'. 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.

但在列 中SQL Profiler显示Batch completed事件的另一个结果：Reads在此处输入图像描述

TLDR： 简单的解释是你试图比较不同的东西。

于 2013-08-17T19:46:12.263 回答