Feedback-ID: ia2694551:Fastmail
Date: Mon, 12 Dec 2022 18:24:07 +0100
From: Alvaro Herrera <alvherre@alvh.no-ip.org>
To: Amit Langote <amitlangote09@gmail.com>
Cc: Robert Haas <robertmhaas@gmail.com>,
	Jacob Champion <jchampion@timescale.com>,
	David Rowley <dgrowleyml@gmail.com>, Tom Lane <tgl@sss.pgh.pa.us>,
	PostgreSQL-development <pgsql-hackers@postgresql.org>
Subject: Re: generic plans and "initial" pruning
Message-ID: <20221212172407.veic522x37kmroli@alvherre.pgsql>
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On 2022-Dec-12, Amit Langote wrote:

> I started feeling like putting all the new logic being added
> by this patch into plancache.c at the heart of GetCachedPlan() and
> tweaking its API in kind of unintuitive ways may not have been such a
> good idea to begin with.  So I started thinking again about your
> GetRunnablePlan() wrapper idea and thought maybe we could do something
> with it.  Let's say we name it GetCachedPlanLockPartitions() and put
> the logic that does initial pruning with the new
> ExecutorDoInitialPruning() in it, instead of in the normal
> GetCachedPlan() path.  Any callers that call GetCachedPlan() instead
> call GetCachedPlanLockPartitions() with either the List ** parameter
> as now or some container struct if that seems better.  Whether
> GetCachedPlanLockPartitions() needs to do anything other than return
> the CachedPlan returned by GetCachedPlan() can be decided by the
> latter setting, say, CachedPlan.has_unlocked_partitions.  That will be
> done by AcquireExecutorLocks() when it sees containsInitialPrunnig in
> any of the PlannedStmts it sees, locking only the
> PlannedStmt.minLockRelids set (which is all relations where no pruning
> is needed!), leaving the partition locking to
> GetCachedPlanLockPartitions().

Hmm.  This doesn't sound totally unreasonable, except to the point David
was making that perhaps we may want this container struct to accomodate
other things in the future than just the partition pruning results, so I
think its name (and that of the function that produces it) ought to be a
little more generic than that.

(I think this also answers your question on whether a List ** is better
than a container struct.)

-- 
Álvaro Herrera         PostgreSQL Developer  —  https://www.EnterpriseDB.com/
"Las cosas son buenas o malas segun las hace nuestra opinión" (Lisias)