Re: First_rows optimization

It's hard to say with MOS and the way that articles get updated with recent "last reviewed" dates even when the information is about versions that are way out of date, but I think the most recent document I've seen said that the only REQUIRED parameter fix was _use_gby_hash_aggregation that had to be set to false because of some code that assumed that "group by" would also result in correct ordering.

Regards
Jonathan Lewis

On Wed, 19 Jan 2022 at 19:17, Pap <oracle.developer35_at_gmail.com> wrote:

> Thank You so much Jonathan and Mark. It helps.
>
> Definitely not seeing any of the docs related to peoplesoft performance,
> stating the parameters like optimizer_mode as 'first_rows_1' and
> optimizer_cost_adj, optimizer_index_caching to set as non default values as
> we have it now.
>
> Will try to search more in the MOS. However if no such things are found ,
> we will probably then plan to move these three global parameters back to
> defaults.
>
>
>
> On Wed, Jan 19, 2022 at 11:27 PM Powell, Mark <mark.powell2_at_dxc.com>
> wrote:
>
>> I know I have seen Peoplesoft Performance and Tuning document recently,
>> but I could not find what I remembered. The following articles while old
>> do contain links to other articles that may still be useful such as tracing
>> and statistics gathering.
>>
>> After Database Upgrade to Oracle Database 19c, Performance Issues with
>> Peoplesoft Complex Queries (Doc ID 2744611.1)
>> E-ORA: PS Query Performance Issues after Oracle Upgrade to 12.1.0.2 (Doc
>> ID 2163277.1)
>> E-ORA: Advice for the Oracle DBA in Supporting PeopleSoft Applications
>> (Doc ID 1445965.1)
>> E-QR: PeopleSoft Query Performance Guidelines & Troubleshooting Tips -
>> Technical Brief (Doc ID 2284809.1)
>> EAR8.8+: Troubleshooting Performance Issues in FSCM PeopleSoft
>> Applications (Doc ID 984534.1)
>> Performance and Tuning for Oracle’s PeopleSoft HCM 9.1 HR Self Service on
>> Oracle Exalogic/Exadata X2-2 Servers (Doc ID 1384242.1)
>>
>> Mark Powell
>> Database Administration
>> (313) 592-5148
>>
>>
>> ------------------------------
>> *From:* oracle-l-bounce_at_freelists.org <oracle-l-bounce_at_freelists.org> on
>> behalf of Jonathan Lewis <jlewisoracle_at_gmail.com>
>> *Sent:* Wednesday, January 19, 2022 12:41 PM
>> *To:* Oracle L <oracle-l_at_freelists.org>
>> *Subject:* Re: First_rows optimization
>>
>> If no-one has mentioned it already - check MOS for the settings demanded
>> by your version of Peoplesoft. You may find that a lot of the settings it
>> used to insist on 10 years ago have been removed.
>>
>> Regarding optimizer_index_caching=80, and the cases where using an index
>> is a bad idea.
>>
>> First_rows_1 (which I think you'd mentioned) makes the arithmetic of
>> costing very messy, and makes the numbers for cost in an execution plan
>> much harder to understand. It has a HUGE impact on making Oracle pick
>> indexes, but also makes Oracle use tablescans if the predicates suggest
>> that there's a LOT of matching data in the table because Oracle then
>> expects to find the first matching row within a few blocks of starting the
>> tablescan.
>>
>> Now add optimizer_index_caching = 80 to the calculation and you find that
>> not only does the optimizer pick a bad strategy, it also isn't very good at
>> telling the difference between a good index and a fairly bad index because
>> it has scaled down the cost of using them so much.
>>
>> Regards
>> Jonathan Lewis
>>
>>
>>
>> On Wed, 19 Jan 2022 at 17:29, Pap <oracle.developer35_at_gmail.com> wrote:
>>
>> 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
>> <https://clicktime.symantec.com/32ynBacZpqKjoCWkc6MVpm47VN?u=https%3A%2F%2Fwww.freelists.org%2Fpost%2Foracle-l%2FSimple-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
>> <https://clicktime.symantec.com/37ZZaS5KwAbdu2gLz6Vv6Pn7VN?u=http%3A%2F%2Fwww.hourim.wordpress.com%2F>
>>
>> Let's Connect -
>> <https://clicktime.symantec.com/3HBXn13i7N2Y4PKekuLnWuN7VN?u=http%3A%2F%2Ffr.linkedin.com%2Fpub%2Fmohamed-houri%2F11%2F329%2F857%2F>*Linkedin
>> Profile
>> <https://clicktime.symantec.com/3HBXn13i7N2Y4PKekuLnWuN7VN?u=http%3A%2F%2Ffr.linkedin.com%2Fpub%2Fmohamed-houri%2F11%2F329%2F857%2F>*
>>
>> My Twitter
>> <https://clicktime.symantec.com/3DA1uoSSF85wWYF9SjMXAGQ7VN?u=https%3A%2F%2Ftwitter.com%2FMohamedHouri>
>> - MohamedHouri
>> <https://clicktime.symantec.com/3DA1uoSSF85wWYF9SjMXAGQ7VN?u=https%3A%2F%2Ftwitter.com%2FMohamedHouri>
>>
>>
>>
>> --
>>
>> Houri Mohamed
>>
>> Oracle DBA-Developer-Performance & Tuning
>>
>> Visit My - Blog
>> <https://clicktime.symantec.com/37ZZaS5KwAbdu2gLz6Vv6Pn7VN?u=http%3A%2F%2Fwww.hourim.wordpress.com%2F>
>>
>> Let's Connect -
>> <https://clicktime.symantec.com/3HBXn13i7N2Y4PKekuLnWuN7VN?u=http%3A%2F%2Ffr.linkedin.com%2Fpub%2Fmohamed-houri%2F11%2F329%2F857%2F>*Linkedin
>> Profile
>> <https://clicktime.symantec.com/3HBXn13i7N2Y4PKekuLnWuN7VN?u=http%3A%2F%2Ffr.linkedin.com%2Fpub%2Fmohamed-houri%2F11%2F329%2F857%2F>*
>>
>> My Twitter
>> <https://clicktime.symantec.com/3DA1uoSSF85wWYF9SjMXAGQ7VN?u=https%3A%2F%2Ftwitter.com%2FMohamedHouri>
>> - MohamedHouri
>> <https://clicktime.symantec.com/3DA1uoSSF85wWYF9SjMXAGQ7VN?u=https%3A%2F%2Ftwitter.com%2FMohamedHouri>
>>
>>
>>
>>

--
http://www.freelists.org/webpage/oracle-l