Re: query performance following 12c upgrade
Date: Thu, 24 Sep 2015 10:44:47 +1000
Message-Id: <E5AC2E35-F44E-4FD7-A1FF-0D4C6CE83E8A_at_burgess-consulting.com.au>
Take a look at the optimizer_adaptive_features and optimizer_dynamic_sampling parameters to limit this type of behaviour in 12.1.0.2. It may be an option to consider if your SQL workload is fairly consistent and non-repetitive - ie more “OLTP” in nature.
We have one customer running with the following values in production for their “OLTP” databases and the behaviour has been good to date:
optimizer_adpative_features=FALSE
optimizer_dynamic_sampling=0
If possible you could implement these settings via ‘alter session’ trigger so you don’t need to set them instance wide.
Regards,
Mark
> On 24 Sep 2015, at 1:23 am, Xiang Rao <xiang_rao_at_hotmail.com> wrote:
>
> We experienced some performance degradations on some databases requiring high traffic and low latency after upgraded to 12c. It turned out the cause was 12c adaptive plans, which spent some time to collect statistics and trying different routes. In addition to the cost of statistics collection and trying different plans, the final plans on some queries were not good, too. The option "ADAPTIVE" with dbms_xplan.display_cursor will show the additional steps used by adaptive plans.
>
> > From: jonathan_at_jlcomp.demon.co.uk <mailto:jonathan_at_jlcomp.demon.co.uk>
> > To: contact_at_soocs.de <mailto:contact_at_soocs.de>; sjb1970_at_gmail.com <mailto:sjb1970_at_gmail.com>
> > CC: oracle-l_at_freelists.org <mailto:oracle-l_at_freelists.org>
> > Subject: RE: query performance following 12c upgrade
> > Date: Wed, 23 Sep 2015 14:57:03 +0000
> >
> >
> > Competition for the result cache sounds viable - though 12 seconds seems a little extreme.
> > If the OP can't get at trace file easily, or query own v$session_event or V$active_session_history then testing 12c after executing
> >
> > alter session set "_optimizer_ads_use_result_cache" = FALSE;
> >
> > might be an indicator - it should disable the use of the result cache for dynamic stats activity.
> >
> >
> >
> > Regards
> > Jonathan Lewis
> > http://jonathanlewis.wordpress.com
> > _at_jloracle
> >
> > ________________________________________
> > From: oracle-l-bounce_at_freelists.org [oracle-l-bounce_at_freelists.org] on behalf of Stefan Koehler [contact_at_soocs.de]
> > Sent: 23 September 2015 15:13
> > To: sjb1970_at_gmail.com
> > Cc: ORACLE-L
> > Subject: Re: query performance following 12c upgrade
> >
> > Hi Steve,
> > thanks for the requested data.
> >
> > The execution plan (especially the important predicate section) and the amount of work is identical - expect the ROWID BATCHED part, but there is an
> > important hint with OPTIMIZER_FEATURES_ENABLE = 12.1.0.2.
> >
> > --------------8<----------------
> > Note
> > -----
> > - dynamic statistics used: dynamic sampling (level=2)
> > - 1 Sql Plan Directive used for this statement
> > --------------8<----------------
> >
> > Is it possible that the query is suffered by latches? How long does the parse itself take? SPDs are based on the result cache and this can have nasty
> > side effects. You can verify this by running a SQL trace on the slow SQL as previously suggested.
> >
> > By the way here are some good references about the result cache and SPD:
> > - http://berxblog.blogspot.de/2015/06/sql-plan-directives-and-result-cache.html
> > - https://dban00b.wordpress.com/2015/04/21/311/
> >
> > Best Regards
> > Stefan Koehler
> >
> > Freelance Oracle performance consultant and researcher
> > Homepage: http://www.soocs.de
> > Twitter: _at_OracleSK
> >
> > > Steve Bradshaw <sjb1970_at_gmail.com> hat am 23. September 2015 um 15:53 geschrieben:
> > >
> > > Hi,
> > >
> > > Please see the attached. opti12 is the results when run with optimizer_featured_enable=12.1.0.2, and opti11 it is 11.2.0.3
> > >
> > > I've had to anonymyse the data/columns/tables etc so the formatting may be a little out.
> > >
> > > Thanks
> > > Steve
> > --
> > http://www.freelists.org/webpage/oracle-l
> >
> >
> > --
> > http://www.freelists.org/webpage/oracle-l
> >
> >
-- http://www.freelists.org/webpage/oracle-lReceived on Thu Sep 24 2015 - 02:44:47 CEST