Re: Really strange performance issue
- From: Andrew Kerber <andrew.kerber@xxxxxxxxx>
- To: "jonathan@xxxxxxxxxxxxxxxxxx" <jonathan@xxxxxxxxxxxxxxxxxx>
- Date: Fri, 31 Oct 2014 09:07:36 -0500
They list two bugs, 13454409 and 16837274.
Sent from my iPad
> On Oct 31, 2014, at 8:48 AM, Jonathan Lewis <jonathan@xxxxxxxxxxxxxxxxxx>
> wrote:
>
>
> Andrew,
>
> If Oracle gave you a visible bug number I'd be interested to see what it was.
>
>
>
> Regards
> Jonathan Lewis
> http://jonathanlewis.wordpress.com
> @jloracle
> From: Andrew Kerber [andrew.kerber@xxxxxxxxx]
> Sent: 31 October 2014 12:37
> To: Jonathan Lewis
> Cc: oracle-l
> Subject: Re: Really strange performance issue
>
> 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@xxxxxxxxxxxxxxxxxx>
> 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
>> @jloracle
>> From: oracle-l-bounce@xxxxxxxxxxxxx [oracle-l-bounce@xxxxxxxxxxxxx] on
>> behalf of Jonathan Lewis [jonathan@xxxxxxxxxxxxxxxxxx]
>> 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
>> @jloracle
>> From: Andrew Kerber [andrew.kerber@xxxxxxxxx]
>> 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@xxxxxxxx> wrote:
>>> +42
>>>
>>> -----Original Message-----
>>> From: oracle-l-bounce@xxxxxxxxxxxxx [mailto:oracle-l-bounce@xxxxxxxxxxxxx]
>>> On Behalf Of Jonathan Lewis
>>> Sent: Thursday, October 30, 2014 4:00 PM
>>> To: andrew.kerber@xxxxxxxxx; 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
>>> @jloracle
>>>
>>> ________________________________________
>>> From: oracle-l-bounce@xxxxxxxxxxxxx [oracle-l-bounce@xxxxxxxxxxxxx] on
>>> behalf of Andrew Kerber [andrew.kerber@xxxxxxxxx]
>>> 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@xxxxxxxxx>
>>> wrote:
>>> >
>>> > Any chance of seeing the Query please?
>>> --
>>> //www.freelists.org/webpage/oracle-l
>>>
>>>
>>> --
>>> //www.freelists.org/webpage/oracle-l
>>>
>>>
>>
>>
>>
>> --
>> Andrew W. Kerber
>>
>> 'If at first you dont succeed, dont take up skydiving.'
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