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 Posted Friday, October 12, 2012 7:22 AM
 SSC Rookie Group: General Forum Members Last Login: Tuesday, November 22, 2016 10:11 AM Points: 45, Visits: 153
 My group is having a discussion as to performance hits when a UDF is used as a macro - IE it accesses no tables, is passed no rows. Some simple logic and a calculation on a date/time value. Here is an abbreviated version of the code (removed check for DST): `Create FUNCTION fnAdjustDateTime (@date datetime, @timezone int)RETURNS datetime ASBEGIN---- declare variables --declare @dst bitdeclare @offset int declare @result datetime ---- Get the timezone data we will need --if @timezone = 1 Select @offset = -5, @dst = 1 else if @timezone = 2 Select @offset = -6, @dst = 1 else if @timezone = 3 Select @offset = -7, @dst = 1 else if @timezone = 4 Select @offset = -8, @dst = 1 else if @timezone = 5 Select @offset = -10, @dst = 0 else if @timezone = 6 Select @offset = -9, @dst = 1 else if @timezone = 7 Select @offset = -4, @dst = 0 else if @timezone = 8 Select @offset = -7, @dst = 0 else if @timezone = 9 Select @offset = -11, @dst = 0 else Select @offset = -6, @dst = 1 ---- Set result now --set @result = DateAdd(hh,@offset,@date) return @result ` Could someone please provide some insight on this as well a a logical explanation ?? Thanks ... Todd
Post #1372111
 Posted Friday, October 12, 2012 7:37 AM
 SSChampion Group: General Forum Members Last Login: Today @ 8:42 AM Points: 14,541, Visits: 38,380
 well, ideally, your scalar UDF can be converted to an inline table funciton instead.`Create FUNCTION fnAdjustDateTime_itv (@date datetime, @timezone int)RETURNS table ASRETURN SELECT DateAdd(hh,CASE WHEN @timezone = 1 THEN -5 WHEN @timezone = 2 THEN -6 WHEN @timezone = 3 THEN -7 WHEN @timezone = 4 THEN -8 WHEN @timezone = 5 THEN -10 WHEN @timezone = 6 THEN -9 WHEN @timezone = 7 THEN -4 WHEN @timezone = 8 THEN -7 WHEN @timezone = 9 THEN -11 END, @date) As ResultDateGOCreate FUNCTION fnAdjustDateTime (@date datetime, @timezone int)RETURNS datetime ASBEGIN---- declare variables --declare @dst bitdeclare @offset int declare @result datetime ---- Get the timezone data we will need --if @timezone = 1 Select @offset = -5, @dst = 1 else if @timezone = 2 Select @offset = -6, @dst = 1 else if @timezone = 3 Select @offset = -7, @dst = 1 else if @timezone = 4 Select @offset = -8, @dst = 1 else if @timezone = 5 Select @offset = -10, @dst = 0 else if @timezone = 6 Select @offset = -9, @dst = 1 else if @timezone = 7 Select @offset = -4, @dst = 0 else if @timezone = 8 Select @offset = -7, @dst = 0 else if @timezone = 9 Select @offset = -11, @dst = 0 else Select @offset = -6, @dst = 1 ---- Set result now --set @result = DateAdd(hh,@offset,@date) return @result END GO`then the calling code gets changed to this:`select name, dbo.fnAdjustDateTime(create_date,1) from sys.objectsselect name, fn.* from sys.objectsCROSS APPLY dbo.fnAdjustDateTime_itv(create_date,1) fn`if you include actual execution palns and compare them, you will start seeing hwo the ITVF version is better; scalar functions usualyl get called once per row,where an ITVF is treated like a set based operationon this small dataset of using sys.objects, my results were the same; but on bigger datasets, the ITVF will be much more efficient. Lowell--help us help you! If you post a question, make sure you include a CREATE TABLE... statement and INSERT INTO... statement into that table to give the volunteers here representative data. with your description of the problem, we can provide a tested, verifiable solution to your question! asking the question the right way gets you a tested answer the fastest way possible!
Post #1372124
 Posted Friday, October 12, 2012 8:25 AM
 SSC-Forever Group: General Forum Members Last Login: Today @ 8:52 AM Points: 42,040, Visits: 39,420
 Todd Young-359443 (10/12/2012)My group is having a discussion as to performance hits when a UDF is used as a macro - IE it accesses no tables, is passed no rows. Some simple logic and a calculation on a date/time value.To add to what Lowell has posted, "One test is worth a thousand expert opinions". Please see the following article for what a difference an "iSF" can make over such a Scalar UDF.http://www.sqlservercentral.com/articles/T-SQL/91724/ --Jeff Moden"RBAR is pronounced "ree-bar" and is a "Modenism" for "Row-By-Agonizing-Row".First step towards the paradigm shift of writing Set Based code: Stop thinking about what you want to do to a row... think, instead, of what you want to do to a column." Helpful Links:How to post code problemsHow to post performance problems
Post #1372171
 Posted Monday, October 15, 2012 11:46 AM
 SSC Rookie Group: General Forum Members Last Login: Tuesday, November 22, 2016 10:11 AM Points: 45, Visits: 153
 I was able to work at home this weekend and found the answer. When calling at UDF that does not access any database table and is used more like a macro, the performance hit is negligible. The raw difference in a 20,000 row dataset was 200 milliseconds. When I removed the overhead of the three DateAdd functions it dropped to 100 milliseconds. We are keeping the function in production and have dropped this as a candidate for optimization.
Post #1372857
 Posted Monday, October 15, 2012 12:09 PM
 SSC-Forever Group: General Forum Members Last Login: Today @ 8:52 AM Points: 42,040, Visits: 39,420
 Todd Young-359443 (10/15/2012)I was able to work at home this weekend and found the answer. When calling at UDF that does not access any database table and is used more like a macro, the performance hit is negligible. The raw difference in a 20,000 row dataset was 200 milliseconds. When I removed the overhead of the three DateAdd functions it dropped to 100 milliseconds. We are keeping the function in production and have dropped this as a candidate for optimization. I guess it really depends on what you call "negligible". You're talking about a function that runs twice as fast and that's not "negligible" especially when you consider that you're only working with 20,000 rows."Mind the pennies and the dollars will take care of themselves." --Jeff Moden"RBAR is pronounced "ree-bar" and is a "Modenism" for "Row-By-Agonizing-Row".First step towards the paradigm shift of writing Set Based code: Stop thinking about what you want to do to a row... think, instead, of what you want to do to a column." Helpful Links:How to post code problemsHow to post performance problems
Post #1372868
 Posted Tuesday, October 16, 2012 7:24 AM
 SSCertifiable Group: General Forum Members Last Login: Today @ 7:54 AM Points: 5,644, Visits: 8,168
 Todd Young-359443 (10/15/2012)I was able to work at home this weekend and found the answer. When calling at UDF that does not access any database table and is used more like a macro, the performance hit is negligible. The raw difference in a 20,000 row dataset was 200 milliseconds. When I removed the overhead of the three DateAdd functions it dropped to 100 milliseconds. We are keeping the function in production and have dropped this as a candidate for optimization. You are fooling yourself. There are MANY reasons why UDFs are bad (see my chapter in the SQL Server MVP Deep Dives 2 book titled Death By UDF and my SQL Rally 2012 session of the same name). Depending on how this UDF gets used it could prevent the optimizer from getting accurate statistics on the query and thus lead to a disastrously bad query plan. And did you know that UDFs also void the use of parallelism? Lots of other potential issues come with them. Best,Kevin G. BolesSQL Server ConsultantSQL MVP 2007-2012TheSQLGuru at GMail
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