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Asked: 2026-05-12T07:44:42+00:00

I’m rewriting some old stored procedure and I’ve come across an unexpected performance issue

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I’m rewriting some old stored procedure and I’ve come across an unexpected performance issue when using a function instead of inline code.

The function is very simple as follow:

ALTER FUNCTION [dbo].[GetDateDifferenceInDays] 
(       
@first_date SMALLDATETIME, 
@second_date SMALLDATETIME
)
RETURNS INT 
AS
BEGIN 

RETURN ABS(DATEDIFF(DAY, @first_date, @second_date))

END

So I’ve got two identical queries, but one uses the function and the other does the calculation in the query itself:

ABS(DATEDIFF(DAY, [mytable].first_date, [mytable].second_date))

Now the query with the inline code runs 3 times faster than the one using the function.

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1 Answer

  1. Editorial Team

    What you have is a scalar UDF ( takes 0 to n parameters and returns a scalar value ). Such UDFs typically cause a row-by-row operation of your query, unless called with constant parameters, with exactly the kind of performance degradation that you’re experiencing with your query.

    See here, here and here for detailed explanations of the peformance pitfalls of using UDFs.

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