"FOR XML Explicit" why poor performance, and any "quick wins" ?

Just started a new job and need a quick fix if possible.
I need to enhance performance of XML data being returned from a SProc which
uses "FOR XML EXPLICIT". (Not something I've done in 10 years as a DBA !)
Front End Application timeouts occasionally occur, which I believe can only
be caused by the SProc creating XML Data.
Without the "For XML Explicit" clause the DB returns about 5000 data rows in
approx 5 seconds.
With the "For XML Explicit" clause, same request will take 3.5 minutes !!
HOWEVER ... If I create copy of the procedure, and run the same request
parameters, it only takes about 6-8 seconds to complete.
No recompile going on as far a I can tell.
No more than 10 calls against this proc per minute.
There are several #Temp tables in the procedure.
I will improve performance by changing the whole process to return record
sets for the front end code to manipulate as required, which will take time
to integrate.
Any ideas on why this "apparent" performance issue happens ?
Any performance tws I can make right now will be of benefit.
Thanks
Steve ...Doing the XML generation on the client-side may not be faster end-to-end
than using the FOR XML EXPLICIT clause.
Here are some questions:
1. You say that simply by copying and thus recompiling the stored proc, you
are getting 6-8 sec execution? But if you run the existing stored proc it
takes 3.5 mins?
If you look at the query plan, do you see any difference between them?
Can you force a recompile of the original stored proc?
It is very unusual that a for xml explicit query runs that much worse
compared to the same query without the FOR XML explicit clause. the overhead
should be more in the 1 sec range for a 5 sec query.
Best regards
Michael
"Steve [DBA-TC]" <Steve [DBA-TC]@.discussions.microsoft.com> wrote in message
news:FD6648EA-CC33-44EA-B7B2-001FEA1139CE@.microsoft.com...
> Just started a new job and need a quick fix if possible.
> I need to enhance performance of XML data being returned from a SProc
> which
> uses "FOR XML EXPLICIT". (Not something I've done in 10 years as a DBA !)
> Front End Application timeouts occasionally occur, which I believe can
> only
> be caused by the SProc creating XML Data.
> Without the "For XML Explicit" clause the DB returns about 5000 data rows
> in
> approx 5 seconds.
> With the "For XML Explicit" clause, same request will take 3.5 minutes !!
> HOWEVER ... If I create copy of the procedure, and run the same request
> parameters, it only takes about 6-8 seconds to complete.
> No recompile going on as far a I can tell.
> No more than 10 calls against this proc per minute.
> There are several #Temp tables in the procedure.
> I will improve performance by changing the whole process to return record
> sets for the front end code to manipulate as required, which will take
> time
> to integrate.
> Any ideas on why this "apparent" performance issue happens ?
> Any performance tws I can make right now will be of benefit.
> Thanks
> Steve ...