On Fri, Oct 18, 2019 at 10:08 AM Amit Kapila <amit.kapila16@gmail.com> wrote:
>
> On Thu, Oct 17, 2019 at 10:51 AM Thomas Munro <thomas.munro@gmail.com> wrote:
> >
> > ===
> > Don't shut down Gather[Merge] early under Limit.
> >
> > Revert part of commit 19df1702f5.
> >
> > Early shutdown was added by that commit so that we could collect
> > statistics from workers, but unfortunately it interacted badly with
> > rescans. Rescanning a Limit over a Gather node could produce a SEGV
> > on 9.6 and resource leak warnings on later releases. By reverting the
> > early shutdown code, we might lose statistics in some cases of Limit
> > over Gather, but that will require further study to fix.
> >
>
> How about some text like below? I have added slightly different text
> to explain the reason for the problem.
>
> "Early shutdown was added by that commit so that we could collect
> statistics from workers, but unfortunately, it interacted badly with
> rescans. The problem is that we ended up destroying the parallel
> context which is required for rescans. This leads to rescans of a
> Limit node over a Gather node to produce unpredictable results as it
> tries to access destroyed parallel context. By reverting the early
> shutdown code, we might lose statistics in some cases of Limit over
> Gather, but that will require further study to fix."
>
> I am not sure but we can even add a comment in the place where we are
> removing some code (in nodeLimit.c) to indicate that 'Ideally we
> should shutdown parallel resources here to get the correct stats, but
> that would lead to rescans misbehaving when there is a Gather [Merge]
> node beneath it. (Explain the reason for misbehavior and the ideas we
> discussed in this thread to fix the same) ........."
>
> I can try to come up with comments in nodeLimit.c on the above lines
> if we think that is a good idea?
>
I have modified the commit message as proposed above and additionally
added comments in nodeLimit.c. I think we should move ahead with this
bug-fix patch. If we don't like the comment, it can anyway be
improved later.
Any suggestions?
--
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com
