内联表值UDF能否优于SELECT列列表中的等效标量UDF吗？

Question

这个问题源于SQLServer:为什么要避免使用表值用户定义的函数？.我开始在一些评论中提问,对我的评论的回复偏离主题.

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因此,您不必阅读整个讨论:我从未听说它说用户定义的函数(UDF)很慢,或者要避免.在上面引用的问题中发布了一些链接,以说明它们很慢.我仍然没有得到它,并要求一个例子.发布了一个示例,性能差异巨大.

我不可能是唯一一个没有意识到可能存在如此大的性能差异的人.我觉得这个事实应该分成一个新的问题和答案,以提高被发现的机会.这是"问题".请不要关闭,因为我想给回答者时间发布答案.

当然,其他人也应该发布答案或例子.我特别感谢任何有助于我理解为什么性能差异如此巨大的东西.

另请注意,我不是在讨论在WHERE子句中使用UDF.我知道这可以阻止优化器完成它的工作.当原始UDF是SELECT列列表的一部分时,我特别感兴趣的是性能差异.

Answer 1

对于基准测试,让我们创建一个包含1M行的表:

CREATE TABLE dbo.Numbers(n INT NOT NULL PRIMARY KEY)
GO
DECLARE @i INT;
SET @i = 1;
INSERT INTO dbo.Numbers(n) SELECT 1;
WHILE @i<1024000 BEGIN
  INSERT INTO dbo.Numbers(n)
    SELECT n + @i FROM dbo.Numbers;
  SET @i = @i * 2;
END;
GO

运行简单的内联添加:

SELECT COUNT(*) FROM(
SELECT n,n+1 AS ValuePlusOne
FROM  dbo.Numbers
) AS t WHERE ValuePlusOne>0

   CPU time = 15 ms, elapsed time = 122 ms.

(1 row(s) affected)
Table 'Numbers'. Scan count 1, logical reads 3521, physical reads 3, read-ahead reads 3498, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

SQL Server Execution Times:
   CPU time = 406 ms,  elapsed time = 951 ms.

创建一个只添加一个整数的标量UDF,并运行1M次:

CREATE FUNCTION dbo.[AddOne] 
(
        @value int
)
RETURNS int
AS
BEGIN
        DECLARE @Result int
        SELECT @Result = @value + 1
        RETURN @Result
END
GO

SELECT COUNT(*) FROM(
SELECT n,dbo.AddOne(n) AS ValuePlusOne
FROM  dbo.Numbers
) AS t WHERE ValuePlusOne>0

   CPU time = 15 ms, elapsed time = 122 ms.

(1 row(s) affected)
Table 'Numbers'. Scan count 1, logical reads 3521, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

SQL Server Execution Times:
   CPU time = 108313 ms,  elapsed time = 295072 ms.

创建内联UDF,其速度与添加速度一样快,并运行1M次:

CREATE FUNCTION dbo.[AddOneInline] 
(
        @value int
)
RETURNS TABLE
AS
RETURN(SELECT @value + 1 AS ValuePlusOne)
GO

SELECT COUNT(*) FROM(
SELECT ValuePlusOne
FROM  dbo.Numbers
CROSS APPLY dbo.[AddOneInline](n)
) AS t WHERE ValuePlusOne>0

SQL Server parse and compile time: 
   CPU time = 0 ms, elapsed time = 35 ms.

(1 row(s) affected)
Table 'Numbers'. Scan count 1, logical reads 3521, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

SQL Server Execution Times:
   CPU time = 391 ms,  elapsed time = 403 ms.

标量UDF与内联UDF的性能差异是显而易见的.