我有一个简单的表值函数，大约需要5秒才能执行。该函数包含一个查询，该查询在1秒内返回数据。我已经阅读了一些博客，据说这可能是由于参数嗅探但尚未找到解决方案。如果由于参数嗅探而如何修复该功能？
System.out.println(p.waitFor());
BufferedReader in = new BufferedReader(new InputStreamReader(p.getErrorStream()));
String resultLine = in.readLine();
while (resultLine != null) {
resultLine = in.readLine();
System.out.println(resultLine);
}
END
答案 0 :(得分：2)
为什么不用单一陈述的TVF？
CREATE FUNCTION [dbo].[fn_PurchaseRecordTESTFIRST]
(
@ID INT = NULL,
@Name nvarchar(MAX),
@PurchaseDate DATE
)
RETURNS TABLE
Return (
SELECT ID
,Name = ProductName
,BasePrice
,Amount = BasePrice*10.25
FROM data.PurchaseRecord i
WHERE i.ID = @ID
AND Date = @PurchaseDate
AND BuyerName=@Name
)
答案 1 :(得分：1)
如果发生参数嗅探，那么你最不担心的是 - 当说瘟疫应该避免使用多语句表值函数（mTVF）时，肖恩就会头疼。根据设计，它们将比内联表值函数（iTVF）慢得多，因为您定义了一个表，填充它，然后返回它。另一方面，iTVF可以被视为接受参数并直接从基础表返回数据的视图。
mTVF的另一个巨大问题是它们会杀死并行性;这意味着如果您有2个CPUS或2,000个CPU，则只有ONE可以解决您的查询。没有例外。看看Jeff Moden的delimitedsplit8K：
CREATE FUNCTION [dbo].[DelimitedSplit8K]
--===== Define I/O parameters
(@pString VARCHAR(8000), @pDelimiter CHAR(1))
--WARNING!!! DO NOT USE MAX DATA-TYPES HERE! IT WILL KILL PERFORMANCE!
RETURNS TABLE WITH SCHEMABINDING AS
RETURN
--===== "Inline" CTE Driven "Tally Table" produces values from 1 up to 10,000...
-- enough to cover VARCHAR(8000)
WITH E1(N) AS (
SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1 UNION ALL
SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1 UNION ALL
SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1
), --10E+1 or 10 rows
E2(N) AS (SELECT 1 FROM E1 a, E1 b), --10E+2 or 100 rows
E4(N) AS (SELECT 1 FROM E2 a, E2 b), --10E+4 or 10,000 rows max
cteTally(N) AS (--==== This provides the "base" CTE and limits the number of rows right up front
-- for both a performance gain and prevention of accidental "overruns"
SELECT TOP (ISNULL(DATALENGTH(@pString),0)) ROW_NUMBER() OVER (ORDER BY (SELECT NULL)) FROM E4
),
cteStart(N1) AS (--==== This returns N+1 (starting position of each "element" just once for each delimiter)
SELECT 1 UNION ALL
SELECT t.N+1 FROM cteTally t WHERE SUBSTRING(@pString,t.N,1) = @pDelimiter
),
cteLen(N1,L1) AS(--==== Return start and length (for use in substring)
SELECT s.N1,
ISNULL(NULLIF(CHARINDEX(@pDelimiter,@pString,s.N1),0)-s.N1,8000)
FROM cteStart s
)
--===== Do the actual split. The ISNULL/NULLIF combo handles the length for the final element when no delimiter is found.
SELECT ItemNumber = ROW_NUMBER() OVER(ORDER BY l.N1),
Item = SUBSTRING(@pString, l.N1, l.L1)
FROM cteLen l;
GO
现在让我们像这样构建一个mTVF版本并进行性能测试......
CREATE FUNCTION [dbo].[DelimitedSplit8K_MTVF]
(@pString VARCHAR(8000), @pDelimiter CHAR(1))
RETURNS @table TABLE (ItemNumber int, Item varchar(100))
AS
BEGIN
--===== "Inline" CTE Driven "Tally Table" produces values from 1 up to 10,000...
-- enough to cover VARCHAR(8000)
WITH E1(N) AS (
SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1 UNION ALL
SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1 UNION ALL
SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1
), --10E+1 or 10 rows
E2(N) AS (SELECT 1 FROM E1 a, E1 b), --10E+2 or 100 rows
E4(N) AS (SELECT 1 FROM E2 a, E2 b), --10E+4 or 10,000 rows max
cteTally(N) AS (--==== This provides the "base" CTE and limits the number of rows right up front
-- for both a performance gain and prevention of accidental "overruns"
SELECT TOP (ISNULL(DATALENGTH(@pString),0)) ROW_NUMBER() OVER (ORDER BY (SELECT NULL)) FROM E4
),
cteStart(N1) AS (--==== This returns N+1 (starting position of each "element" just once for each delimiter)
SELECT 1 UNION ALL
SELECT t.N+1 FROM cteTally t WHERE SUBSTRING(@pString,t.N,1) = @pDelimiter
),
cteLen(N1,L1) AS(--==== Return start and length (for use in substring)
SELECT s.N1,
ISNULL(NULLIF(CHARINDEX(@pDelimiter,@pString,s.N1),0)-s.N1,8000)
FROM cteStart s
)
--===== Do the actual split. The ISNULL/NULLIF combo handles the length for the final element when no delimiter is found.
INSERT @table
SELECT ItemNumber = ROW_NUMBER() OVER(ORDER BY l.N1),
Item = SUBSTRING(@pString, l.N1, l.L1)
FROM cteLen l;
RETURN;
END
GO
在继续之前，我想谈谈@John Cappelletti的陈述：
在[关于MAX数据类型]之前我已经看过这样的声明，但我还没有看到任何引人注目的统计数据
对于一些引人注目的统计数据，让我们对iTVF版本的delimitedSplit8K做一个小调整，并将输入字符串更改为varchar（max）：
CREATE FUNCTION [dbo].[DelimitedSplit8K_VCMAXINPUT]
(@pString VARCHAR(max), @pDelimiter CHAR(1))
RETURNS TABLE WITH SCHEMABINDING AS
RETURN
--===== "Inline" CTE Driven "Tally Table" produces values from 1 up to 10,000...
-- enough to cover VARCHAR(8000)
WITH E1(N) AS (
SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1 UNION ALL
SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1 UNION ALL
SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1
), --10E+1 or 10 rows
E2(N) AS (SELECT 1 FROM E1 a, E1 b), --10E+2 or 100 rows
E4(N) AS (SELECT 1 FROM E2 a, E2 b), --10E+4 or 10,000 rows max
cteTally(N) AS (--==== This provides the "base" CTE and limits the number of rows right up front
-- for both a performance gain and prevention of accidental "overruns"
SELECT TOP (ISNULL(DATALENGTH(@pString),0)) ROW_NUMBER() OVER (ORDER BY (SELECT NULL)) FROM E4
),
cteStart(N1) AS (--==== This returns N+1 (starting position of each "element" just once for each delimiter)
SELECT 1 UNION ALL
SELECT t.N+1 FROM cteTally t WHERE SUBSTRING(@pString,t.N,1) = @pDelimiter
),
cteLen(N1,L1) AS(--==== Return start and length (for use in substring)
SELECT s.N1,
ISNULL(NULLIF(CHARINDEX(@pDelimiter,@pString,s.N1),0)-s.N1,8000)
FROM cteStart s
)
--===== Do the actual split. The ISNULL/NULLIF combo handles the length for the final element when no delimiter is found.
SELECT ItemNumber = ROW_NUMBER() OVER(ORDER BY l.N1),
Item = SUBSTRING(@pString, l.N1, l.L1)
FROM cteLen l;
GO
现在我们有三个版本的功能：原始的iTVF，一个接受varchar（max）和一个mTVF版本。现在进行性能测试。
-- sample data
IF OBJECT_ID('tempdb..#string') IS NOT NULL DROP TABLE #string;
SELECT TOP (10000)
id = IDENTITY(int, 1,1),
txt = REPLICATE(newid(), ABS(checksum(newid())%5)+1)
INTO #string
FROM sys.all_columns a, sys.all_columns b;
SET NOCOUNT ON;
-- Performance tests:
PRINT 'ITVF 8K'+char(13)+char(10)+replicate('-',90);
GO
DECLARE @st datetime2 = getdate(), @x varchar(20);
SELECT @x = ds.Item
FROM #string s
CROSS APPLY dbo.DelimitedSplit8K(s.txt, '-') ds;
PRINT datediff(ms, @st, getdate());
GO 5
PRINT 'MTVF 8K'+char(13)+char(10)+replicate('-',90);
GO
DECLARE @st datetime2 = getdate(), @x varchar(20);
SELECT @x = ds.Item
FROM #string s
CROSS APPLY dbo.DelimitedSplit8K_MTVF(s.txt, '-') ds;
PRINT datediff(ms, @st, getdate());
GO 5
PRINT 'ITVF VCMAX'+char(13)+char(10)+replicate('-',90);
GO
DECLARE @st datetime2 = getdate(), @x varchar(20);
SELECT @x = ds.Item
FROM #string s
CROSS APPLY dbo.DelimitedSplit8K_VCMAXINPUT(s.txt, '-') ds;
PRINT datediff(ms, @st, getdate());
GO 5
和结果：
ITVF 8K
------------------------------------------------------------------------------------------
Beginning execution loop
280
267
284
300
280
Batch execution completed 5 times.
MTVF 8K
------------------------------------------------------------------------------------------
Beginning execution loop
1190
1190
1157
1173
1187
Batch execution completed 5 times.
ITVF VCMAX
------------------------------------------------------------------------------------------
Beginning execution loop
1204
1220
1190
1190
1203
Batch execution completed 5 times.
采用varchar（max）的mTVF和iTVF版本都慢了4-5倍。再次：避免像瘟疫一样的mTVF，并尽可能避免使用最大数据类型。