Re: Library lock issue

I have faced a similar 12cR1 issue of parallel slave (PX) refusing to share the execution plan of their query coordinator (QC) despite both QC and its PX slaves were in the same instance. It turned to be a kind of bug because of the usage of bind variable (yes using bind variables !!) and dynamic sampling.

 But here you are using literals and GTT with private statistics. Bear in mind that when you are using session private statistics Oracle will not propagate GTT private statistics from one session to another one. It achieves this by invalidating similar cursor between sessions.

https://hourim.wordpress.com/2019/07/30/global-temporary-table-private-statistic <https://hourim.wordpress.com/2019/07/30/global-temporary-table-private-statistics/>

That’s said I haven’t tested how Oracle will consider the QC coordinator session per regards to its PX slave sessions in case of private GTT sessions.

You can see if your execution plan has the following Note signaling that it has used a GTT private statistics or not

Note

• Global temporary table session private statistics used

And use Tanel Poder nonshared sql script to get the reason for which PX slaves are refusing the share the execution plan of their QC

Best Regards

Mohamed Houri

Le mar. 2 févr. 2021 à 09:42, Chinar Aliyev <chinaraliyev_at_gmail.com> a écrit :

> Hi,
> There are several bugs in Oracle Support and might one of them be
> appropriate for your case.
>
> It happens when QC and PX Slaves are allocated in different instances, and
> a remote slave has to parse the statement based on the info sent by QC
> (to achieve the same execution plan generated by QC).
> To reduce parsing you can force it as Sayan has mentioned. Also, check
> support notes/bugs.
>
> Best Regards
>
> On Tue, Feb 2, 2021 at 11:12 AM Pap <oracle.developer35_at_gmail.com> wrote:
>
>> Thanks a lot.
>>
>> We will definitely try rerunning by setting session level
>> "parallel_force_local" parameter to true as we set this value as 'FALSE' in
>> v$parameter.
>>
>> *"parallel slaves will not be able to join the execution because they are
>> not able to reproduce the same execution plan as the coordinator. "*
>>
>> I am a bit confused with above statement on the dynamic sampling part, I
>> see out of three global temporary tables used in this query , two of them
>> are using private session level stats as it's gathered inside the code. But
>> one(RTNI) is having stats set as NULL and also its locked , which means the
>> dynamic sampling must have been triggered for that table only. But in that
>> case too, i am not able to understand how the dynamic sampling can be the
>> cause, can you please explain bit more. As because , my understanding is
>> parsing will happen at the first stage and till that time parallel slaves
>> wont get involve in real execution. So as the table related info will be
>> available in the node-3 as that being the session of the query
>> coordinator/parent session , so that should only do the parsing work.
>> Please correct me if wrong.
>>
>> Regards
>> Pap
>>
>> On Tue, Feb 2, 2021 at 11:20 AM Laurentiu Oprea <
>> laurentiu.oprea06_at_gmail.com> wrote:
>>
>>> Hello,
>>>
>>> Additional to what has been mentioned, usually this issue is caused by
>>> the fact that parallel slaves will not be able to join the execution
>>> because they are not able to reproduce the same execution plan as the
>>> coordinator.
>>> Dynamic Sampling is often one of the root causes for this issue, If DS
>>> is not helping in your case you can create a spl patch for that sql with a
>>> lower level of DS or even disable it : dynamic_sampling(0).
>>>
>>> Good luck
>>>
>>> În lun., 1 feb. 2021 la 23:53, Sayan Malakshinov <xt.and.r_at_gmail.com> a
>>> scris:
>>>
>>>> Hi Pap,
>>>>
>>>> Have you tried to set parallel_force_local=true? Your QC is on Node 3
>>>> while your slaves are on Node 2:
>>>> 1. Parallel queries usually work much better if they don't need to send
>>>> data between nodes and fight for concurrent access to the same data;
>>>> 2. You are using global temporary tables, so their data is private for
>>>> your session and stored on the same node as your session. In case of
>>>> parallel access to GTT, QC has to send also their segment info, so your
>>>> slaves have to request data from it, ie from Node 3.
>>>> 3. Also that means that node 2 have to parse your query too for your
>>>> slaves (sometimes it even leads to more child cursors)
>>>>
>>>> On Mon, Feb 1, 2021 at 10:28 PM Pap <oracle.developer35_at_gmail.com>
>>>> wrote:
>>>>
>>>>> Hello All, We are seeing some odd behaviour. Its version 12.1.0.2.0 of
>>>>> oracle. And a small query(finishing in <1 minutes) which is executing in
>>>>> parallel(2) is experiencing "library cache lock" and "cursor: pin S wait on
>>>>> X" between its own slaves. I mean to say the blocking session is appearing
>>>>> as its own slave sessions. We have "parallel_degree_policy" set as MANUAL
>>>>> in v$parametr. This query is running for different literals one after
>>>>> another multiple times in a loop fashion. And all these samples logged in
>>>>> dba_hist_active_sess_history showing IN_PARSE as 'Y'. Dueto these waits the
>>>>> overall execution time of the process is going beyond ~5hrs+. The CPU and
>>>>> IO waits as noted in sql monitor is very small. Wondering how parallel
>>>>> slave processes of the same query are blocking each other during parsing
>>>>> itself. Or are we hitting any bug in this version?
>>>>>
>>>>> Attached is the sql and its run time sql monitor. And all the tables
>>>>> used in this query are global temporary tables "on commit preserve row"
>>>>> types.
>>>>>
>>>>>
>>>>>
>>>>> Thanks And Regards
>>>>>
>>>>> Pap
>>>>>
>>>>
>>>>
>>>> --
>>>> Best regards,
>>>> Sayan Malakshinov
>>>> Oracle performance tuning engineer
>>>> Oracle ACE Associate
>>>> http://orasql.org
>>>>
>>>
>
> --
> *Chinar Aliyev*
>
>
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>
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>
>

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