Re: First_rows optimization

Thank you Mohamed.

One more thing I just noticed, this peoplesoft database also has optimizer_index_caching= 80, and optimizer_index_cost_adj= 50. Is this non default setup is something peoplesoft product suggested or it must have been done by the team only in the past?

And also we noticed many queries going for index paths here whereas full scan is making those faster. Can this be totally a manually setup which is causing such symptoms and so we should revert thee back to defaults?

On Tue, Jan 18, 2022 at 6:23 PM Mohamed Houri <mohamed.houri_at_gmail.com> wrote:

> Pap,
>
> *However, not able to get it fully , when you said "** of course, you
> should delete these altered sessions wherever you find them." . Do you mean
> to say , we need to set it back to "all_rows" after the intended
> query/session finishes with "first_rows_1" optimization mode? Please
> correct me if I'm wrong*.
>
> In those queries where you have noticed slowness, print the corresponding
> outline and, *if* you find the hint FIRST_ROWS in this outline, and you
> manage to correlate this with an INDEX FULL SCAN operation which takes
> the most time, *then *you have to intervene in the corresponding batch
> and get rid of the alter session set optimizer_mode=first_rows_1; from
> this batch.
>
> Best regards
>
> Mohamed
>
> Le mar. 18 janv. 2022 à 12:26, Pap <oracle.developer35_at_gmail.com> a
> écrit :
>
>> Thank You So much. This database is on version 19.9 , but OFE is still
>> 11.2.0.4.
>>
>> So it means at least the optimizer mode as 'first_row_1' has not to be
>> set as global but rather should be done only in specific cases on session
>> level if required. And thus, this non default value is not a guidance from
>> peoplesoft product.
>>
>> However, not able to get it fully , when you said *"** of course, you
>> should delete these altered sessions wherever you find them." . *Do you
>> mean to say , we need to set it back to "all_rows" after the intended
>> query/session finishes with "first_rows_1" optimization mode? Please
>> correct me if I'm wrong.
>>
>> On Tue, Jan 18, 2022 at 1:35 PM Mohamed Houri <mohamed.houri_at_gmail.com>
>> wrote:
>>
>>> Pap
>>>
>>> Regarding PeopleSoft and its recommendations concerning certain
>>> parameters of the CBO, we've talked about it a lot here on this list, so
>>> I'd definitely like to clarify that from my experience with at least 3
>>> different applications using PeopleSoft.
>>>
>>> Firstly, the optimizer_mode= first_rows_1 parameter is not set at a
>>> global level but only via a couple of altered sessions. And yes, of course,
>>> you should delete these altered sessions wherever you find them. This is
>>> exactly what I did for my 3 clients.
>>>
>>> Second, setting the _optimizer_skip_scan_enabled parameter to FALSE is
>>> indeed suggested by PeopleSoft. I set it to TRUE globally in the first days
>>> of testing in UAT. And you should do the same.
>>>
>>> Finally, PeopleSoft suggests changing the _unnest_subquery parameter
>>> to FALSE. In this case, it's up to you. In a 12.2 version I set it to TRUE
>>> and I had to intervene on several queries to set it locally to FALSE. You
>>> can follow my example or do the opposite. You are free. But I'm sure
>>> you will whatever value you’ll adopt for this parameter, I'm sure that
>>> you will have to intervene locally to force the opposite value. in 19.3 I
>>> adopted the opposite strategy
>>>
>>> You can follow the PeopleSoft suggestion concerning the
>>> _gby_hash_aggregation_enabled parameter
>>>
>>> Best regards
>>>
>>> Mohamed
>>>
>>> Le mar. 18 janv. 2022 à 08:54, Lok P <loknath.73_at_gmail.com> a écrit :
>>>
>>>> You have not mentioned the current version of this Oracle database ?
>>>> Not sure of first_rows_1, however , I saw one thread in past as below,
>>>> in which "Mohamed Houri" mentioned that those three parameters were exactly
>>>> the ones suggested by peoplesoft to be changed :). So it might be that
>>>> first_row_1 is also peoplesoft suggested only and it may not be advisable
>>>> to play with that. Others may comment on this.
>>>>
>>>>
>>>> https://www.freelists.org/post/oracle-l/Simple-query-opting-higher-cost-path
>>>>
>>>> On Tue, Jan 18, 2022 at 10:41 AM Pap <oracle.developer35_at_gmail.com>
>>>> wrote:
>>>>
>>>>> Hello Listers, We have a peoplesoft application database and almost
>>>>> all the time we see the queries in the backend as above ~2000-3000+ lines
>>>>> with multiple UNIONS in them. And the estimation seems really bad making us
>>>>> confused many times but then we see the optimizer_mode has been set as
>>>>> first_rows_1. Thus the indexed path is favored all the time even though few
>>>>> of the queries perform better by forcing a full scan path. So I wanted to
>>>>> check with experts here if those are really the recommended setups for
>>>>> peoplesoft databases in general and thus should not be changed? During
>>>>> encountering any performance issue many times we don't get the sql monitors
>>>>> because of such long queries.
>>>>>
>>>>> Along with above there are other parameters like _unnest_subquery,
>>>>> _gby_hash_aggregation_enabled, _optimizer_skip_scan_enabled etc are set as
>>>>> non default i.e. false.
>>>>>
>>>>> Regards
>>>>> Pap
>>>>>
>>>>
>>>
>>> --
>>>
>>> Houri Mohamed
>>>
>>> Oracle DBA-Developer-Performance & Tuning
>>>
>>> Visit My - Blog <http://www.hourim.wordpress.com/>
>>>
>>> Let's Connect - <http://fr.linkedin.com/pub/mohamed-houri/11/329/857/>*Linkedin
>>> Profile <http://fr.linkedin.com/pub/mohamed-houri/11/329/857/>*
>>>
>>> My Twitter <https://twitter.com/MohamedHouri> - MohamedHouri
>>> <https://twitter.com/MohamedHouri>
>>>
>>>
>
> --
>
> Houri Mohamed
>
> Oracle DBA-Developer-Performance & Tuning
>
> Visit My - Blog <http://www.hourim.wordpress.com/>
>
> Let's Connect - <http://fr.linkedin.com/pub/mohamed-houri/11/329/857/>*Linkedin
> Profile <http://fr.linkedin.com/pub/mohamed-houri/11/329/857/>*
>
> My Twitter <https://twitter.com/MohamedHouri> - MohamedHouri
> <https://twitter.com/MohamedHouri>
>
>

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