Seriously Quirky Performance Behavior in a Query

SQLServerCentral is supported by Red Gate Software Ltd.

Popular Topics

Home

Members

Calendar

Who's On

Home » SQL Server 2008 » SQL Server 2008 - General » Seriously Quirky Performance Behavior in a...

Seriously Quirky Performance Behavior in a Query

Rate Topic

Display Mode

Topic Options

Author

Message

pat.baggett

Posted Wednesday, December 19, 2012 8:55 AM

Forum Newbie

Group: General Forum Members
Last Login: Thursday, February 14, 2013 6:59 AM
Points: 4, Visits: 12

I have come across a strange phenomenon when querying data from one of our historical databases and was wondering if someone could please provide some sort of explanation?

I am seeing an enormous performance hit when I execute a query containing a literal date range (embedded in the query) as opposed to using variables.

As an example, I have 2 queries:

Query 1
select count(a.[counter]), sum(a.[clmtotal]) from [direct].dbo.[claims] a with (nolock)
inner join [direct].dbo.[pclaim] c with (nolock) on c.counter = a.claimlink
where a.[type] = 'P'
and c.[hubid] = 1
and c.billdate between '10/1/2012' and '11/30/2012'

Execution time varies anywhere from 20 - 30 minutes (or more)

Query 2
declare @begdate datetime, @enddate datetime
select @begdate = '10/1/2012', @enddate = '11/30/2012'
select count(a.[counter]), sum(a.[clmtotal]) from [direct].dbo.[claims] a with (nolock)
inner join [direct].dbo.[pclaim] c with (nolock) on c.counter = a.claimlink
where a.[type] = 'P'
and c.[hubid] = 1
and c.billdate between @begdate and @enddate

Execution time averages around 5 -10 seconds

Why the difference? I can easily change my code to use the "variable approach", but I would really like an explanation as to this enormous difference as it may help to increase performance in other projects.

Thanks in advance.

Post Attachments

variables.sqlplan (6 views, 20.38 KB)

no variables.sqlplan (6 views, 18.55 KB)

Post #1398473

anthony.green

Posted Wednesday, December 19, 2012 8:59 AM

SSCertifiable

Group: General Forum Members
Last Login: Friday, April 12, 2013 3:51 AM
Points: 5,075, Visits: 4,831

My first guess would be with all the implicit conversions it has to do converting the strings '10/1/2012' and '11-30-2012' to datetime.

Can you upload the execution plans in a SQLPLAN format so we can see the differences between the two?

The link in my signature on posting performance problems will help if you are unsure.

Want an answer fast? Try here
How to post data/code for the best help - Jeff Moden
Need a string splitter, try this - Jeff Moden
How to post performance problems - Gail Shaw
CrossTabs-Part1 & Part2 - Jeff Moden
SQL Server Backup, Integrity Check, and Index and Statistics Maintenance - Ola Hallengren
Managing Transaction Logs - Gail Shaw
Troubleshooting SQL Server: A Guide for the Accidental DBA - Jonathan Kehayias and Ted Krueger

Post #1398476

pat.baggett

Posted Wednesday, December 19, 2012 9:08 AM

Forum Newbie

Group: General Forum Members
Last Login: Thursday, February 14, 2013 6:59 AM
Points: 4, Visits: 12

done - see attached.

Your explanation makes perfect sense though. Sometimes the answer is staring you right in the face - but you still need someone to point it out. w00t

Thanks for the quick reply.

Post #1398487

GSquared

Posted Wednesday, December 19, 2012 9:13 AM

SSCoach

Group: General Forum Members
Last Login: 2 days ago @ 1:55 PM
Points: 15,442, Visits: 9,571

On the one with the string literals, try '20121130' and '20121001' instead of the US-formatted dates and see what happens.

- Gus "GSquared", RSVP, OODA, MAP, NMVP, FAQ, SAT, SQL, DNA, RNA, UOI, IOU, AM, PM, AD, BC, BCE, USA, UN, CF, ROFL, LOL, ETC
Property of The Thread

"Nobody knows the age of the human race, but everyone agrees it's old enough to know better." - Anon

Post #1398493

GilaMonster

Posted Wednesday, December 19, 2012 9:21 AM

SSC-Dedicated

Group: General Forum Members
Last Login: Today @ 7:38 AM
Points: 37,726, Visits: 29,987

Parameter sniffing, or lack thereof. Probably combined with stale statistics so that SQL gets a completely incorrect row estimate when it can sniff the 'parameters' (in this case string literals) while the variables, because they can't be sniffed give a rougher estimate that's not so likely to be affected by stale stats.

http://sqlinthewild.co.za/index.php/2007/11/27/parameter-sniffing/
http://sqlinthewild.co.za/index.php/2008/02/25/parameter-sniffing-pt-2/

Look at the two estimated plans posted (actuals would be far better), look at the row estimations. Without the actual plans there's no way to see which of those estimates are more accurate, but I'll bet that the 5000 is closer than the 1.

p.s. Watch those nolocks. http://blogs.msdn.com/b/davidlean/archive/2009/04/06/sql-server-nolock-hint-other-poor-ideas.aspx

Gail Shaw
Microsoft Certified Master: SQL Server 2008, MVP
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability

We walk in the dark places no others will enter
We stand on the bridge and no one may pass

Post #1398498

Dave Ballantyne

Posted Wednesday, December 19, 2012 9:28 AM

SSCommitted

Group: General Forum Members
Last Login: Friday, May 10, 2013 4:07 PM
Points: 1,943, Visits: 8,227

I think your statistics are out of date on that table.
Update them , then run the queries again (using WITH RECOMPILE) and see what the plans look like.

The estimations and therefore the plans are vastly different, you would normally expect a query with literal value to be faster, but it estimates at 1 row.

Clear Sky SQL
My Blog
Kent user group

Post #1398503

GilaMonster

Posted Wednesday, December 19, 2012 9:33 AM

SSC-Dedicated

Group: General Forum Members
Last Login: Today @ 7:38 AM
Points: 37,726, Visits: 29,987

Dave Ballantyne (12/19/2012)

Update them , then run the queries again (using WITH RECOMPILE) and see what the plans look like.

No need to use recompile. The stats update alone would invalidate the plan.

Gail Shaw
Microsoft Certified Master: SQL Server 2008, MVP
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability

We walk in the dark places no others will enter
We stand on the bridge and no one may pass

Post #1398507

Dave Ballantyne

Posted Wednesday, December 19, 2012 9:53 AM

SSCommitted

Group: General Forum Members
Last Login: Friday, May 10, 2013 4:07 PM
Points: 1,943, Visits: 8,227

GilaMonster (12/19/2012)

Dave Ballantyne (12/19/2012)

Update them , then run the queries again (using WITH RECOMPILE) and see what the plans look like.

No need to use recompile. The stats update alone would invalidate the plan.

Tru dat :)

Clear Sky SQL
My Blog
Kent user group

Post #1398518

« Prev Topic | Next Topic »

Permissions