Re: Locking issue

Thank You Pap.

Support is suggesting patch but still want to have a second thought from experts here, if it's okay to set the 'real time stats' option OFF in the 19C instances considering this type of row cache lock issues?

Regards
Lok

On Fri, Jun 18, 2021 at 8:31 AM Pap <oracle.developer35_at_gmail.com> wrote:

> I think the two parameters (_optimizer_use_stats_on_conventional &
> _dml_optimizer_gather_stats_on conventional_dml ) Dominic mentioned to
> control the real-time stats were altered at system level only. Not seeing
> anything which can stop real time stats at table level.
>
> There exists another hint no_gather_optimizer_statistics which controls
> the real time stats collection. But yes then you have to put it in each and
> every dml statement which is happening on this impacted table. That doesn't
> look convenient though :).
>
> The real time stats feature does look tempting but the question you asked
> on whether we should turn off the feature fully on 19c instance considering
> the buggy nature ,is something should be decided by comparing the pros Vs
> cons or cost Vs benefit of this feature. And I will leave to the experts
> here who have played with many 19C instances which I have not done so far.
>
>
>
> On Thu, 17 Jun 2021, 8:53 pm Lok P, <loknath.73_at_gmail.com> wrote:
>
>>
>> Thank you Jonathan.
>>
>> As far as my knowledge, we have not done any special treatment for this
>> table but If I see dba_tab_statistics , I see two entries as below for the
>> same table and out of which one entry says about the
>> STATS_ON_CONVENTIONAL_DML. So not sure but it seems it's automatically
>> picked up and also seems like it(realtime stats collection) should have
>> been controllable at table level by someway.
>>
>> However, considering this realtime statistics feature looks tempting one
>> as it helps to have the stats accurate even during run time which will help
>> in making the optimizer equipped with better data pattern information, so
>> the team is a bit reluctant to get rid of this feature fully. However, at
>> the same time, in one of our cases here we saw the bad side of it i.e. how
>> a simple "SELECT * "query is asking a dictionary cache lock because of the
>> real time stats feature causing concurrency issues throughout.
>>
>> We have multiple other 19C instances running , So thinking of correct
>> approach here, is it advisable setting this feature fully OFF at the
>> database level as Dominic suggested through
>> "_optimizer_use_stats_on_conventional & _dml_optimizer_gather_stats_on
>> conventional_dml" for all of those? Or should we better get the
>> patch(32042352) applied as oracle suggested and let the feature be ON i.e.
>> default in database level?
>>
>>
>> Table_name num_rows empty_blocks chain_cnt avg_row_len sample_size
>> last_analyzed global_stats user_stats notes
>> P 738578220 0 0 248 738578220 6/11/2021 102511PM YES NO
>> P 739679871 6/17/2021 109011PM STATS_ON_CONVENTIONAL_DML
>>
>> On Thu, Jun 17, 2021 at 3:09 PM Jonathan Lewis <jlewisoracle_at_gmail.com>
>> wrote:
>>
>>> Lok P,
>>>
>>> The 19.10 RU backports a parameter optimizer_real_time_statistics
>>> according to this note (
>>> https://blogs.oracle.com/optimizer/real-time-statistics-parameter ) by
>>> Nigel Bayliss, with the default value of FALSE so on your next upgrade it
>>> seems likely that the feature will disable itself automatically - which
>>> suggests it would be safe to disable it manually.
>>>
>>> I don't have access to a 19.9, so I assume that it is currently a hidden
>>> parameter but maybe it's more subtle than that.
>>> There's also the problem that your query is reporting the use of
>>> real-time statisics, but the collection of the statistics is from DML, so
>>> maybe you have to find a way to disable the entire realtime statistics
>>> feature to bypass your dictionary cache activity.
>>>
>>> Regards
>>> Jonathan Lewis
>>>
>>>
>>>
>>> On Thu, 17 Jun 2021 at 06:10, Lok P <loknath.73_at_gmail.com> wrote:
>>>
>>>> Actually I am not sure if we can relate our issue to exactly the same
>>>> one which you pointed above. Our Db version is 19.9.0.0.0. In our case it
>>>> is a simple SELECT query ( SELECT /*+ FULL(P) +*/ * FROM P ) doing
>>>> FTS on a table (holding 700million rows) from multiple sessions(20+
>>>> sessions) and were experiencing DC lock on 'dc_realtime_colst/tabst' and
>>>> all the other new session accessing same object 'P' were gets locked in
>>>> parse stage(cursor pin s wait on X) there after creating a big storm of
>>>> concurrency. Support is pointing to the below bug and suggesting the patch
>>>> to install to fix this bug.
>>>>
>>>> 32042352
>>>> <https://support.oracle.com/epmos/faces/PatchDetail?requestId=24138539&_afrLoop=252185249564103&patchId=32042352&_afrWindowMode=0&_adf.ctrl-state=168q4vx617_93#> HIGH
>>>> ROW CACHE MUTEX WAITS ON 19.8 WITH HIGH CONCURRENCY
>>>> <https://support.oracle.com/epmos/faces/PatchDetail?requestId=24138539&_afrLoop=252185249564103&patchId=32042352&_afrWindowMode=0&_adf.ctrl-state=168q4vx617_93#>
>>>>
>>>>
>>>>
>>>> I had one related question, as this all seems to be related to 'dynamic
>>>> statistics/stats on conventional DML option' as that is visible in the note
>>>> section in the SELECT query plan. So is it advisable to switch that off for
>>>> this sql/table someway without negatively impacting it anything?
>>>>
>>>>
>>>>
>>>> On Wed, Jun 9, 2021 at 9:21 PM Dominic Brooks <dombrooks_at_hotmail.com>
>>>> wrote:
>>>>
>>>>> Out of interest, but off on a tangent from original thread, I am also
>>>>> hitting what seems to be some oddities around this dc_realtime_colst.
>>>>>
>>>>>
>>>>>
>>>>> Trying to drop a 230GB partitioned table it’s taking hours with
>>>>> notable waits on DLM cross inst call completion and notable row cache lock
>>>>> waits on dc_realtime_colst (and dc_histogram_defs).
>>>>>
>>>>> For my issue, investigation is pointing towards two notes which point
>>>>> at same bug:
>>>>>
>>>>> *Drop User Cascade Command Hang On "DLM cross inst call completion"
>>>>> (Doc ID 2671064.1)*
>>>>>
>>>>> *Bug 31208287 - Deleting Enqueues For A Large Partitioned Table Hangs
>>>>> In KJCI_WAIT/KJCI_PROCESSCRQ (Doc ID 31208287.8)*
>>>>>
>>>>>
>>>>>
>>>>> WORKAROUND:
>>>>>
>>>>> Drop in smaller portions. For instance first remove stats/histograms
>>>>> from the
>>>>>
>>>>> columns.
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> Sent from Mail <https://go.microsoft.com/fwlink/?LinkId=550986> for
>>>>> Windows 10
>>>>>
>>>>>
>>>>>
>>>>> *From: *Noveljic Nenad <nenad.noveljic_at_vontobel.com>
>>>>> *Sent: *28 May 2021 08:57
>>>>> *To: *Lok P <loknath.73_at_gmail.com>
>>>>> *Cc: *ORACLE-L (oracle-l_at_freelists.org) <oracle-l_at_freelists.org>
>>>>> *Subject: *RE: Locking issue
>>>>>
>>>>>
>>>>>
>>>>> Yes, this bug should be fixed in 19.9 in the base release. I can tell
>>>>> from my installation that it’s definitely fixed in 19.9.0.0.201020.
>>>>>
>>>>>
>>>>>
>>>>> You can verify it for your patch level:
>>>>>
>>>>>
>>>>>
>>>>> $ORACLE_HOME/OPatch/opatch lsinventory | grep 31414023
>>>>>
>>>>>
>>>>>
>>>>> We still don’t know the reason for row cache lock id=62/63
>>>>> allocations. Unfortunately, the 10222 trace doesn’t spool the call stacks
>>>>> for cid=62/63, only for cid=16/8.
>>>>>
>>>>>
>>>>>
>>>>> We’d need to capture the call stacks for 62 or 63 to find out more.
>>>>>
>>>>>
>>>>>
>>>>> If you want to go down that rabbit hole, I can prepare a DTrace script
>>>>> if you’re on Solaris or gdb if you’re on Linux. The later would be better
>>>>> to run on a test system if you’re unexperienced with gdb.
>>>>>
>>>>>
>>>>>
>>>>> Let me know.
>>>>>
>>>>>
>>>>>
>>>>> Best regards,
>>>>>
>>>>>
>>>>>
>>>>> Nenad
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> *From:* Lok P <loknath.73_at_gmail.com>
>>>>> *Sent:* Freitag, 28. Mai 2021 06:05
>>>>> *To:* Noveljic Nenad <nenad.noveljic_at_vontobel.com>
>>>>> *Cc:* ORACLE-L (oracle-l_at_freelists.org) <oracle-l_at_freelists.org>
>>>>> *Subject:* Re: Locking issue
>>>>>
>>>>>
>>>>>
>>>>> Thank you Nenad. We have oracle version 19.9.0.0. Need to check patch
>>>>> details. But it seems from the bug details , this bug should have been
>>>>> fixed by this version. correct me if wrong.
>>>>>
>>>>>
>>>>>
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>>>>

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