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From: Robert Haas <robertmhaas@gmail.com>
Date: Mon, 7 Aug 2023 12:25:52 -0400
Message-ID: <CA+TgmoY1C4emO+nJj_BTotjiJ9m7asWX0QeMWxNPKpTGysji0w@mail.gmail.com>
Subject: Re: generic plans and "initial" pruning
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Amit Langote <amitlangote09@gmail.com>, Thom Brown <thom@linux.com>, 
	Daniel Gustafsson <daniel@yesql.se>, Alvaro Herrera <alvherre@alvh.no-ip.org>, 
	Andres Freund <andres@anarazel.de>, David Rowley <dgrowleyml@gmail.com>, 
	Jacob Champion <jchampion@timescale.com>, PostgreSQL Hackers <pgsql-hackers@postgresql.org>
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On Mon, Aug 7, 2023 at 11:44=E2=80=AFAM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Right, I doubt that changing that is going to work out well.
> Hash joins might have issues with it too.

I thought about the case, because Hash and Hash Join are such closely
intertwined nodes, but I don't see any problem there. It doesn't
really look like it would matter in what order things got cleaned up.
Unless I'm missing something, all of the data structures are just
independent things that we have to get rid of sometime.

> Could it work to make the patch force child cleanup before parent,
> instead of after?  Or would that break other places?

To me, it seems like the overwhelming majority of the code simply
doesn't care. You could pick an order out of a hat and it would be
100% OK. But I haven't gone and looked through it with this specific
idea in mind.

> On the whole though I think it's probably a good idea to leave
> parent nodes in control of the timing, so I kind of side with
> your later comment about whether we want to change this at all.

My overall feeling here is that what Gather and Gather Merge is doing
is pretty weird. I think I kind of knew that at the time this was all
getting implemented and reviewed, but I wasn't keen to introduce more
infrastructure changes than necessary given that parallel query, as a
project, was still pretty new and I didn't want to give other hackers
more reasons to be unhappy with what was already a lot of very
wide-ranging change to the system. A good number of years having gone
by now, and other people having worked on that code some more, I'm not
too worried about someone calling for a wholesale revert of parallel
query. However, there's a second problem here as well, which is that
I'm still not sure what the right thing to do is. We've fiddled around
with the shutdown sequence for parallel query a number of times now,
and I think there's still stuff that doesn't work quite right,
especially around getting all of the instrumentation data back to the
leader. I haven't spent enough time on this recently enough to be sure
what if any problems remain, though.

So on the one hand, I don't really like the fact that we have an
ad-hoc recursion arrangement here, instead of using
planstate_tree_walker or, as Amit proposes, a List. Giving subordinate
nodes control over the ordering when they don't really need it just
means we have more code with more possibility for bugs and less
certainty about whether the theoretical flexibility is doing anything
in practice. But on the other hand, because we know that at least for
the Gather/GatherMerge case it seems like it probably matters
somewhat, it definitely seems appealing not to change anything as part
of this patch set that we don't really have to.

I've had it firmly in my mind here that we were going to need to
change something somehow -- I mean, the possibility of returning in
the middle of node initialization seems like a pretty major change to
the way this stuff works, and it seems hard for me to believe that we
can just do that and not have to adjust any code anywhere else. Can it
really be true that we can do that and yet not end up creating any
states anywhere with which the current cleanup code is unprepared to
cope? Maybe, but it would seem like rather good luck if that's how it
shakes out. Still, at the moment, I'm having a hard time understanding
what this particular change buys us.

--=20
Robert Haas
EDB: http://www.enterprisedb.com