Re: Multiple SQL_IDs....why?

From: joel garry <joel-garry_at_home.com>
Date: Wed, 24 Feb 2010 13:45:35 -0800 (PST)
Message-ID: <87914d22-fb02-4f81-a89a-13dd2c91bd10_at_t34g2000prm.googlegroups.com>

On Feb 24, 10:52�am, Guy Peleg <makleeengineer..._at_gmail.com> wrote:
> On Feb 23, 12:14�pm, "Michel Cadot" <micadot{at}altern{dot}org> wrote:
>
>
>
> > "Guy Peleg" <makleeengineer..._at_gmail.com> a crit dans le message de news:
> > 513e35ea-858b-4b3f-bc51-270b2e6bb..._at_j6g2000vbd.googlegroups.com...
> > On Feb 23, 4:15 pm, "Michel Cadot" <micadot{at}altern{dot}org> wrote:
>
> > > "Guy Peleg" <makleeengineer..._at_gmail.com> a crit dans le message de news:
> > > e7b40657-4e16-4079-936b-7be353697..._at_s17g2000vbs.googlegroups.com...
> > > | Oracle 10.2.0.3 on solaris, running a select query on a test machine
> > > | takes one second, execution
> > > | plan shows that index range scan is used. On the production node same
> > > | statement takes more than
> > > | a minute and execution plan shows full table scan.
> > > |
> > > | On the production node, looking at V$SQL I can see that I have two
> > > | execution plans for the query, one
> > > | seems fast fast with index range scan and the slow one with full table
> > > | scan.
> > > |
> > > | I'm trying to understand why I have multiple versions of the same
> > > | statement.
> > > |
> > > | Any ideas?
> > > |
> > > |
> > > | SQL> select
> > > | sql_id,child_number,executions,optimizer_cost,optimizer_mode from v
> > > | $sql where sql_id='79tg4h3uhwncc';
> > > |
> > > | SQL_ID CHILD_NUMBER EXECUTIONS OPTIMIZER_COST OPTIMIZER_
> > > | ------------- ------------ ---------- -------------- ----------
> > > | 79tg4h3uhwncc 0 94 49 ALL_ROWS
> > > | 79tg4h3uhwncc 1 60 4716 ALL_ROWS
> > > |
> > > | SQL> select child_number, bind_mismatch B, optimizer_mode_mismatch O
> > > | from v$sql_shared_cursor
> > > | 2 where sql_id='79tg4h3uhwncc';
> > > |
> > > | CHILD_NUMBER B O
> > > | ------------ - -
> > > | 0 N N
> > > | 1 N N
> > > |
> > > |
> > > | Not shown here but BIND_DATA, PARSING_SCHEMA_NAME are the same.
> > > |
> > > | Regards,
> > > |
> > > | Guy Peleg
> > > | Maklee Engineering
>
> > > There are about 40 mismatch columns, you queried only 2,
> > > have a look at the other ones.
>
> > > Regards
> > > Michel
>
> > ROLL_INVALIDATE_MISMATCH is set to Y, is it possible to determine why
> > the execution plan
> > has changed?
>
> > SQL> �select * from v$sql_shared_cursor where sql_id='79tg4h3uhwncc';
>
> > SQL_ID � � � �ADDRESS � � � � �CHILD_ADDRESS � �CHILD_NUMBER U S O O S
> > L S E B P I S T A B D L T R I I R L I O S M U T N F A I T D L
>
> > ------------- ---------------- ---------------- ------------ - - - - -
> > - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
>
> > D B P C S R P T M B M R O P M F L
> > - - - - - - - - - - - - - - - - -
> > 79tg4h3uhwncc 0000000532754E90 0000000532753F60 � � � � � �0 N N N N N
> > N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N
>
> > N N N N N N N N N N N N N N N N N
>
> > 79tg4h3uhwncc 0000000532754E90 000000051BDF2DC0 � � � � � �1 N N N N N
> > N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N
>
> > N N N N N N N N N N N Y N N N N N
>
> > ---------------------------------------------------------------------------�-------------------
>
> > Read the following thread, abovie last but one Dion Cho's post.http://forums.oracle.com/forums/thread.jspa?threadID=592771
>
> > Regards
> > Michel- Hide quoted text -
>
> > - Show quoted text -
>
> Something still does not make sense to me. I understand that the
> execution plan was being invalidated,
> most probably after statistics was collected at 10pm.
>
> The database was started Monday morning. Throughout Monday, it was
> using the fast execution plan, Tuesday it
> switched to the slow execution plan and today it switched back to the
> fast version. The table this query operate against
> has 700K rows and it may grow by ~10,000 per day. So why would we
> switch back and forth between these plans?

That sounds like classic ol' bind peeking. See http://www.pythian.com/news/867/stabilize-oracle-10gs-bind-peeking-behaviour-by-cutting-histograms/

Perhaps your data distribution is such that you are near a discontinuity with a slight change in query? How exactly are you gathering statistics?

jg

--
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Received on Wed Feb 24 2010 - 15:45:35 CST