From: Tom Lane <tgl@sss.pgh.pa.us>
To: Andres Freund <andres@anarazel.de>
Cc: Pavel Stehule <pavel.stehule@gmail.com>
Cc: Amit Langote <amitlangote09@gmail.com>
Cc: PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Subject: Re: plan cache overhead on plpgsql expression
Date: Thu, 26 Mar 2020 15:05:20 -0400
Message-ID: <14781.1585249520@sss.pgh.pa.us> (raw)
In-Reply-To: <20200326184947.vniqm2kqa4hmv56x@alap3.anarazel.de>
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Andres Freund <andres@anarazel.de> writes:
> On 2020-03-26 14:37:59 -0400, Tom Lane wrote:
>> + * This function, together with CachedPlanIsSimplyValid, provides a fast path
>> + * for revalidating "simple" generic plans. The core requirement to be simple
>> + * is that the plan must not require taking any locks, which translates to
>> + * not touching any tables; this happens to match up well with an important
>> + * use-case in PL/pgSQL.
> Hm - is there currently sufficient guarantee that we absorb sinval
> messages? Would still matter for types, functions, etc?
There are potentially issues of that sort throughout the backend, not
just here, since we don't have any locking on types or functions.
I don't think it's this patch's job to address that. In practice
I think we've thought about it and concluded that the cost/benefit
of introducing such locks just isn't promising:
* Generally speaking you can't do anything very interesting to a type
anyway, at least not with supported DDL. The worst-case situation that
could materialize AFAIK is possibly evaluating slightly-stale constraints
for a domain. (The typcache does have sinval invalidation for those
constraints, but I don't recall offhand how much we guarantee about
how quickly we'll notice updates.)
* For functions, you might execute a somewhat stale version of the
function body. The bad effects there are pretty limited since a function
is defined by just one catalog row, unlike tables, so you can't see a
self-inconsistent version of it.
The amount of lock overhead that it would take to remove those edge
cases seems slightly staggering, so I doubt we'd ever do it.
> While it'd do a small bit unnecessary work, I do wonder if it'd be
> better to use this code in ResourceOwnereReleaseInternal().
When and if we refactor to expose this sort of thing more generally,
it might be worth doing it like that. I don't think it helps much
right now.
> Perhaps add a reference to the new (appreciated, btw) DO comment above?
Can do.
Again, thanks for reviewing!
regards, tom lane
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To: pgsql-hackers@postgresql.org
Cc: tgl@sss.pgh.pa.us, andres@anarazel.de, pavel.stehule@gmail.com, amitlangote09@gmail.com
Subject: Re: plan cache overhead on plpgsql expression
In-Reply-To: <14781.1585249520@sss.pgh.pa.us>
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