Re: Explain buffers wrong counter with parallel plans

From: Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: Andres Freund <andres(at)anarazel(dot)de>, "Jonathan S(dot) Katz" <jonathan(dot)katz(at)excoventures(dot)com>, Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>, Adrien Nayrat <adrien(dot)nayrat(at)anayrat(dot)info>, PostgreSQL mailing lists <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Explain buffers wrong counter with parallel plans
Date: 2018-08-03 08:39:23
Message-ID: CAA4eK1KuC1qVAU4V67ym0iGBRcJNtB80nOMo33pdnzayKciE1g@mail.gmail.com
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On Thu, Aug 2, 2018 at 11:14 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> On Thu, Aug 2, 2018 at 5:41 AM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>> I have created three patches (a) move InstrStartParallelQuery from its
>> original location so that we perform it just before ExecutorRun (b)
>> patch to fix the gather stats by calling shutdown at appropriate place
>> and allow stats collection in ExecShutdownNode (c) Probit calling
>> ExecShutdownNode if there is a possibility of backward scans (I have
>> done some basic tests with this patch, if we decide to proceed with
>> it, then some more verification and testing would be required).
>>
>> I think we should commit first two patches as that fixes the problem
>> being discussed in this thread and then do some additional
>> verification for the third patch (mentioned in option c). I can
>> understand if people want to commit the third patch before the second
>> patch, so let me know what you guys think.
>
> I'm happy with the first two patches.
>

Thanks. I have pushed those two patches.

> In the third one, I don't think
> "See ExecLimit" is a good thing to put a comment like this, because
> it's too hard to find the comment to which it refers, and because
> future commits are likely to edit or remove that comment without
> noticing the references to it from elsewhere. Instead I would just
> write, in all three places, /* If we know we won't need to back up, we
> can release resources at this point. */ or something like that.
>

Okay, I have changed the comment as per your suggestion in the
attached patch. I will do some more testing/verification of this
patch and will commit over the weekend or on Monday if everything is
fine.

I have noticed that part of the comment atop ExecShutdownNode is now
redundant. See attached edit_comments_shutdown_node_v1, let me know
if you think otherwise.

--
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com

Attachment Content-Type Size
prohibit_shutdown_backward_scans_v2.patch application/octet-stream 2.3 KB
edit_comments_shutdown_node_v1.patch application/octet-stream 781 bytes

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