Re: Really strange performance issue
From: Andrew Kerber <andrew.kerber_at_gmail.com>
Date: Fri, 31 Oct 2014 07:37:11 -0500
Message-Id: <BDC5F82E-5A45-409C-B6B6-84B7CAFBD585_at_gmail.com>
Jonathan - you are correct on the oddities, but there is no unique constraint. The row is unique however, and there are only about 30 rows in the table. I think I have any typos corrected below.
Sent from my iPad
>> On Thu, Oct 30, 2014 at 6:44 PM, Mark W. Farnham <mwf_at_rsiz.com> wrote:
>> +42
>>
>> -----Original Message-----
>> From: oracle-l-bounce_at_freelists.org [mailto:oracle-l-bounce_at_freelists.org]
>> On Behalf Of Jonathan Lewis
>> Sent: Thursday, October 30, 2014 4:00 PM
>> To: andrew.kerber_at_gmail.com; Howard Latham
>> Cc: oracle-l
>> Subject: RE: Really strange performance issue
>>
>>
>>
>> Although we generally expect cardinality feedback to result in better plans
>> it's possible that a change in plan could change the order in which the data
>> driving (e.g.) a scalar subquery is accessed, increasing the number of times
>> a subquery is executed without changing the number of rows returned in the
>> rowsource. If by "embedded select" you actually mean a scalar subquery it's
>> possible that the main query does look more efficient to the optimizer, but
>> the scalar subquery runs far more time. Easy to detect if you enable
>> rowsource execution statistics (e.g. add hint gather_plan_statistics) and
>> use the 'allstats last' format option with dbms_xplan.display_cursor().
>>
>>
>>
>>
>>
>> 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 Andrew Kerber [andrew.kerber_at_gmail.com]
>> Sent: 30 October 2014 14:51
>> To: Howard Latham
>> Cc: oracle-l
>> Subject: Re: Really strange performance issue
>>
>> I'll have to see if I can remove identifying information, but there is
>> really nothing special about it, basically a two table join with a couple of
>> embedded selects to get a date range. The plan is the same in both cases.
>>
>> Sent from my iPad
>>
>> > On Oct 30, 2014, at 9:42 AM, Howard Latham <howard.latham_at_gmail.com>
>> wrote:
>> >
>> > Any chance of seeing the Query please?
>> --
>> http://www.freelists.org/webpage/oracle-l
>>
>>
>> --
>> http://www.freelists.org/webpage/oracle-l
Date: Fri, 31 Oct 2014 07:37:11 -0500
Message-Id: <BDC5F82E-5A45-409C-B6B6-84B7CAFBD585_at_gmail.com>
Jonathan - you are correct on the oddities, but there is no unique constraint. The row is unique however, and there are only about 30 rows in the table. I think I have any typos corrected below.
Oracle support says it is a bug, no patch available yet, and the work around is to set _optimizer_use_feedback=false. Now I need to figure out the ramifications of doing that.
> SELECT 'Data4', > wdata.created, > wdata.value2 > FROM wdata, wbedata > WHERE wbedata.my_number = '888888' > AND (wdata.created <= (select trim(value_string) from other.parm_value where job_type = 'D TEST' and parm_type = 'B END') || '-23.59.59.999999' > AND (wdata.created >= (select trim(value_string) from other.parm_value where job_type = 'D TEST' and parm_type = 'B START') || '-23.59.59.999999')) > AND (wdata.created = wbedata.created) > AND (wdata.value2 = wbedata.value2) > AND (wdata.value3 = wbedata.value3) > ORDER BY wdata.created;
Sent from my iPad
> On Oct 31, 2014, at 4:15 AM, Jonathan Lewis <jonathan_at_jlcomp.demon.co.uk> wrote: > > > > I'm assuming, by the way, that any oddities in the sample code are just the result of trying to edit out the confidential stuff. > > In particular: I've asssumed that there's a unique constraint on (job_type, parm_type) so that the optimizer can "know" that there's only a single possible value; and I've assumed that the subquery is written to supply the column type and hasn't thrown in another obfuscating factor by causing column conversion. > > > 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 Jonathan Lewis [jonathan_at_jlcomp.demon.co.uk] > Sent: 31 October 2014 08:57 > To: Andrew Kerber; Mark W. Farnham > Cc: Howard Latham; oracle-l > Subject: RE: Really strange performance issue > > > Looking at the code, I think Sayan's comment is the relevant one. > > Waving my hands and guessing WILDLY - but I suspect I could create a data set where this happens: > > The optimizer is probably handling your subqueries as "unknown constant", which gives you a range scan on unknown values which gives the optimizer a guess of 0.25% - hence the application of cardinality feedback. > On the first pass the optimizer drives off the created date - and discovers that it does a lot more work than expected (more rows), so on the second pass it reverses the join, which turns out to be a bad idea because the optimizer's estimated cardinality of '88888' (which doesn't get modified by the first pass) is badly wrong and/or the chosen access path back into wdata is much less efficient than expected. > > > Regards > Jonathan Lewis > http://jonathanlewis.wordpress.com > _at_jloracle > From: Andrew Kerber [andrew.kerber_at_gmail.com] > Sent: 31 October 2014 01:32 > To: Mark W. Farnham > Cc: Jonathan Lewis; Howard Latham; oracle-l > Subject: Re: Really strange performance issue > > Below is a heavily redacted version of the query, all columns names and values changed,as I said it is pretty straightforward, > > SELECT 'Data4', > wdata.created, > wdata.value2 > FROM wdata, wbedata > WHERE wbedata.my_number = '888888' > AND (wdata.created <= (select trim(value_string) from other.parm_value where job_type = 'D TEST' and parm_type = 'B END') || '-23.59.59.999999' > AND (wdata.created >= (select trim(value_string) from other.parm_value where job_type = 'D TEST' and parm_type = 'B START') || '-23.59.59.999999')) > AND (wdata.created = wbedata.created) > AND (wdata.value2 = wbedata.value2) > AND (wdata.value3 = wbedata.value3) > ORDER BY wdata.created; >
>> On Thu, Oct 30, 2014 at 6:44 PM, Mark W. Farnham <mwf_at_rsiz.com> wrote:
>> +42
>>
>> -----Original Message-----
>> From: oracle-l-bounce_at_freelists.org [mailto:oracle-l-bounce_at_freelists.org]
>> On Behalf Of Jonathan Lewis
>> Sent: Thursday, October 30, 2014 4:00 PM
>> To: andrew.kerber_at_gmail.com; Howard Latham
>> Cc: oracle-l
>> Subject: RE: Really strange performance issue
>>
>>
>>
>> Although we generally expect cardinality feedback to result in better plans
>> it's possible that a change in plan could change the order in which the data
>> driving (e.g.) a scalar subquery is accessed, increasing the number of times
>> a subquery is executed without changing the number of rows returned in the
>> rowsource. If by "embedded select" you actually mean a scalar subquery it's
>> possible that the main query does look more efficient to the optimizer, but
>> the scalar subquery runs far more time. Easy to detect if you enable
>> rowsource execution statistics (e.g. add hint gather_plan_statistics) and
>> use the 'allstats last' format option with dbms_xplan.display_cursor().
>>
>>
>>
>>
>>
>> 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 Andrew Kerber [andrew.kerber_at_gmail.com]
>> Sent: 30 October 2014 14:51
>> To: Howard Latham
>> Cc: oracle-l
>> Subject: Re: Really strange performance issue
>>
>> I'll have to see if I can remove identifying information, but there is
>> really nothing special about it, basically a two table join with a couple of
>> embedded selects to get a date range. The plan is the same in both cases.
>>
>> Sent from my iPad
>>
>> > On Oct 30, 2014, at 9:42 AM, Howard Latham <howard.latham_at_gmail.com>
>> wrote:
>> >
>> > Any chance of seeing the Query please?
>> --
>> http://www.freelists.org/webpage/oracle-l
>>
>>
>> --
>> http://www.freelists.org/webpage/oracle-l
> > > > -- > Andrew W. Kerber > > 'If at first you dont succeed, dont take up skydiving.'
-- http://www.freelists.org/webpage/oracle-lReceived on Fri Oct 31 2014 - 13:37:11 CET