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SQL Server: 局部變量是如何影響查詢性能的

發(fā)布時間: 2012/8/4 17:04:55

數(shù)據(jù)庫開發(fā)者在存儲過程和腳本中使用局部變量是很常見的事情，但是，局部變量會影響查詢的性能，接下來我們來證實這一點。

首先讓我們創(chuàng)建一個表并插入一些測試數(shù)據(jù)：

USE AdventureWorks
GO
CREATE TABLE TempTable
(tempID UNIQUEIDENTIFIER,tempMonth INT, tempDateTime DATETIME )
GO
INSERT INTO TempTable (tempID, tempMonth, tempDateTime)
SELECT NEWID(),(CAST(100000*RAND() AS INT) % 12) + 1 ,GETDATE()
GO 100000 -- (EXECUTE THIS BATCH 100000 TIME)
-- Create an index to support our query
CREATE NONCLUSTERED INDEX [IX_tempDateTime] ON [dbo].[TempTable]
([tempDateTime] ASC)
INCLUDE ( [tempID]) WITH ( ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
GO

然后我們做一個簡單的查詢：

SET STATISTICS IO ON
GO
SELECT * FROM TempTable
WHERE tempDateTime > '2012-07-10 03:18:01.640'

Table 'TempTable'. Scan count 1, logical reads 80, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

檢查這個執(zhí)行計劃以及索引檢索的屬性，你會發(fā)現(xiàn)預估行數(shù)是實際行數(shù)的兩倍，但并不會太影響執(zhí)行計劃，因為優(yōu)化器選擇了最合適的查詢方法：

查詢優(yōu)化器根據(jù)基本統(tǒng)計直方圖來預估數(shù)據(jù)行數(shù)，即：EQ_ROWS + AVG_RANGE_ROWS (77 + 88.64286) DBCC SHOW_STATISTICS ('dbo.TempTable', IX_tempDateTime)

現(xiàn)在我們修改 SELECT 語句以使用局部變量，你會發(fā)現(xiàn)查詢優(yōu)化器使用了一個不同的查詢計劃，這是一個更耗時的計劃，為什么？

DECLARE @RequiredDate DATETIME
SET @RequiredDate = '2012-07-10 03:18:01.640'
SELECT * FROM TempTable
WHERE tempDateTime > @RequiredDate

------------------------------------------------------------------------------------------

Table 'TempTable'. Scan count 1, logical reads 481, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.