Re: Library lock issue

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
>>>
>>

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