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From: Amit Langote <amitlangote09@gmail.com>
Date: Fri, 23 Aug 2024 21:48:27 +0900
Message-ID: <CA+HiwqHNb9jrwOFHfmASfiGc=SnqXs7THwQ_Rta=z+ognYV8qw@mail.gmail.com>
Subject: Re: generic plans and "initial" pruning
To: Robert Haas <robertmhaas@gmail.com>
Cc: Alvaro Herrera <alvherre@alvh.no-ip.org>, Andres Freund <andres@anarazel.de>, 
	Daniel Gustafsson <daniel@yesql.se>, David Rowley <dgrowleyml@gmail.com>, 
	PostgreSQL Hackers <pgsql-hackers@postgresql.org>, Thom Brown <thom@linux.com>, 
	Tom Lane <tgl@sss.pgh.pa.us>
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On Wed, Aug 21, 2024 at 10:10=E2=80=AFPM Robert Haas <robertmhaas@gmail.com=
> wrote:
> On Wed, Aug 21, 2024 at 8:45=E2=80=AFAM Amit Langote <amitlangote09@gmail=
.com> wrote:
> > * The replanning aspect of the lock-in-the-executor design would be
> > simpler if a CachedPlan contained the plan for a single query rather
> > than a list of queries, as previously mentioned. This is particularly
> > due to the requirements of the PORTAL_MULTI_QUERY case. However, this
> > option might be impractical.
>
> It might be, but maybe it would be worth a try? I mean,
> GetCachedPlan() seems to just call pg_plan_queries() which just loops
> over the list of query trees and does the same thing for each one. If
> we wanted to replan a single query, why couldn't we do
> fake_querytree_list =3D list_make1(list_nth(querytree_list, n)) and then
> call pg_plan_queries(fake_querytree_list)? Or something equivalent to
> that. We could have a new GetCachedSinglePlan(cplan, n) to do this.

I've been hacking to prototype this, and it's showing promise. It
helps make the replan loop at the call sites that start the executor
with an invalidatable plan more localized and less prone to
action-at-a-distance issues. However, the interface and contract of
the new function in my prototype are pretty specialized for the replan
loop in this context=E2=80=94meaning it's not as general-purpose as
GetCachedPlan(). Essentially, what you get when you call it is a
'throwaway' CachedPlan containing only the plan for the query that
failed during ExecutorStart(), not a plan integrated into the original
CachedPlanSource's stmt_list. A call site entering the replan loop
will retry the execution with that throwaway plan, release it once
done, and resume looping over the plans in the original list. The
invalid plan that remains in the original list will be discarded and
replanned in the next call to GetCachedPlan() using the same
CachedPlanSource. While that may sound undesirable, I'm inclined to
think it's not something that needs optimization, given that we're
expecting this code path to be taken rarely.

I'll post a version of a revamped locks-in-the-executor patch set
using the above function after debugging some more.

--
Thanks, Amit Langote