What if your UDF is a scalar detereministic UDF with a constant value passed in? How many times is this UDF executed:

CREATE FUNCTION dbo.AbsoluteValue (@i INT)

RETURNS INT

WITH SCHEMABINDING

AS

BEGIN

RETURN ABS(@i);

END;

GO

SELECT dbo.AbsoluteValue(-10)

FROM sys.columns;

GO

If SQL Server is executing a deterministic UDF with a constant parameter hundreds or thousands of times, it is seriously wasting resources.