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From: Jakub Wartak <jakub.wartak@enterprisedb.com>
Date: Tue, 20 Jan 2026 10:19:48 +0100
Message-ID: <CAKZiRmxmhjnKp-GMvdL-2jE-nic5g7-TLdsveirrbRv2zdSdyQ@mail.gmail.com>
Subject: Re: pg_plan_advice
To: Robert Haas <robertmhaas@gmail.com>
Cc: Lukas Fittl <lukas@fittl.com>, Jacob Champion <jacob.champion@enterprisedb.com>, 
	Dian Fay <di@nmfay.com>, Matheus Alcantara <matheusssilv97@gmail.com>, 
	PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
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Precedence: bulk

On Mon, Jan 19, 2026 at 9:00=E2=80=AFPM Robert Haas <robertmhaas@gmail.com>=
 wrote:
>
> On Mon, Jan 19, 2026 at 5:53=E2=80=AFAM Jakub Wartak
> <jakub.wartak@enterprisedb.com> wrote:
> >    a) v10-0001 - any example producing such a dummy subplan? (whatever
> > I've tried I cannot come up with one)
[..]
> EXPLAIN SELECT * FROM random() UNION SELECT * FROM random() WHERE false;

Oh well, that was easy, thanks! Now I see `RTI 3 (function,
in-from-clause): /  Subplan: setop_2 (dummy)`

I don't have any further insights on v10-[124] other than mentioned earlier=
.

> >    c) In v10-0004, maybe in pathnodes.h we could use typedef enum rathe=
r than
> >       list of #defines? (see attached)
>
> I personally hate that style and I think Andres loves it. Whee!

Oh, ok, nvm, but while two of You we are at this, vim or emacs ? ;)
/me ducks & covers

> > 4. Some raw perf numbers on non-assert builds (please ignore +/- 3%
> > jumps), it just hurts
> >    in one scenario where oq2 drops like 9% of juice (quite expected, it=
's not
> >    an issue to be, just posting full results)
> >
> > tps                           oq1  oq2    oq3  oq4
> > master                        41   14745  439  435
> > master+v10-000[1-4]           42   15055  439  432
> > master+v10full                41   14734  429  437
> > master+v10full+loaded         42   15014  442  438
> > master+v10full+loaded+advice  41   13481  424  439
> >
> > (same but in percentages)
> > %tps_to_master                oq1  oq2    oq3  oq4
> > master                        100  100    100  100
> > master+v10-000[1-4]           102  102    100  99
> > master+v10full                100  100    98   100
> > master+v10full+loaded         102  102    101  101
> > master+v10full+loaded+advice  100  91     97   101
>
> I think these numbers look pretty good. I mean, there is obviously
> room for improvement. We should look at where the CPU cycles are going
> in the oq2 case and try to optimize. But even without that, it's not
> terrible, IMHO.
>
> > So out of curiosity the oq2 on 1 CPU core behavior looks like below:
> > - no advices --> ~1000 TPS
> > - enabled pg_plan_advice.advice to lengthy, but unrelated thing and it
> > gets ~890TPS
>
> I'm not sure exactly where the CPU cycles are going here, but one
> known problem is that we have to re-parse the advice string for every
> query. This thread discusses the challenges of creating some
> infrastructure that would allow us to avoid that:
>
> http://postgr.es/m/f87504a6-9dfd-4467-89de-84232cb54f72@gmail.com
>
> Maybe I should start thinking about other ways to avoid that overhead,

Meh...

> because that thread doesn't seem to be progressing much, but maybe the
> reparsing isn't even the main problem.
> > - in both cases (empty and set) the bottleneck seems to in palloc0, but
> >     empty plan_advice: it's more like palloc0() <- newNode() <-
> > create_index_path()
> >     <- build_index_paths()
> >     with plan_advice set: palloc0() <- newNode() <- create_nestloop_pat=
h() ..
>
> I've also seen some palloc-related issues with the patch -- it has to
> build some data structures and that does palloc stuff -- but there
> shouldn't really be any difference in the code paths you show here.
> That's just core code, which should be doing the same thing either way
> if the advice is not relevant.

Yeah, in both it looks like memory allocation and lots of newNode()
called , quite expected.

> > - so if anything people should not put something there blindly, but jus=
t SET
> >   and RESET afterwards (unless we get pinning of SQL plan id to advices=
) as
> >   this might have cost in high-TPS scenarios.
>
> Yes, I think that's definitely a potential issue. I'd like the
> overhead of this module to be as low as possible, but it's bound to
> have at least some overhead, and people will have to decide whether
> it's worth it.

I think we should simply ignore, and maybe later just note the fact this is
not free with a single sentence in some docs for 0005. I was just curious o=
f the
impact and this was measured using pure EXPLAIN (so without query execution=
 to
measure impact of non-empty pg_plan_advice), I'm assuming that in
properly managed
systems execution part will always dominate the workload anyway and
one should be
using prepared statements anyway.

-J.