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In-Reply-To: 
 <CAPmGK14CFGgCSCor1DHe6GVVQpjj+zDq755K5_vgc32U2+OGYQ@mail.gmail.com>
From: Amit Langote <amitlangote09@gmail.com>
Date: Wed, 24 Jun 2026 20:20:29 +0900
Message-ID: 
 <CA+HiwqFd61sXgUWijKg8DfRQDtwYEGSdxuEPDb8rq22rxzxN_g@mail.gmail.com>
Subject: Re: BUG #19484: Segmentation fault triggered by FDW
To: Etsuro Fujita <etsuro.fujita@gmail.com>
Cc: Richard Guo <guofenglinux@gmail.com>,
 Matheus Alcantara <matheusssilv97@gmail.com>,
	Ayush Tiwari <ayushtiwari.slg01@gmail.com>,
 Rafia Sabih <rafia.pghackers@gmail.com>,
	798604270@qq.com, pgsql-bugs@lists.postgresql.org,
	Amit Langote <amitlan@postgresql.org>
Content-Type: multipart/mixed; boundary="00000000000029411c0654fe1141"
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Fujita-san,

On Wed, Jun 24, 2026 at 6:15=E2=80=AFPM Etsuro Fujita <etsuro.fujita@gmail.=
com> wrote:
> On Wed, Jun 24, 2026 at 1:25=E2=80=AFPM Amit Langote <amitlangote09@gmail=
.com> wrote:
> > On Wed, Jun 24, 2026 at 11:23=E2=80=AFAM Richard Guo <guofenglinux@gmai=
l.com> wrote:
> > > On Wed, Jun 24, 2026 at 10:43=E2=80=AFAM Amit Langote <amitlangote09@=
gmail.com> wrote:
> > > > If we=E2=80=99d rather avoid it, I have a no-field alternative for =
REL_18: show_modifytable_info() is the only reader of the re-indexed list, =
and it can recompute the mapping from node->resultRelations and node->fdwPr=
ivLists, both plan-ordered and untouched by pruning.
> > >
> > > This sounds like a nice solution.  Since show_modifytable_info() is
> > > only used for EXPLAIN, I guess the recompute overhead on the
> > > re-indexed list should be fine.
> >
> > Thanks for chiming in, here is the patch to do so.
>
> Thanks for the patch!
>
> +           /*
> +            * node->fdwPrivLists is indexed by the original, pre-pruning
> +            * result relation order and is parallel to node->resultRelat=
ions.
> +            * Initial pruning may have dropped earlier relations, so the=
 kept
> +            * index j need not match the original position; find this
> +            * relation's entry by its range table index instead.
> +            */
> +           forboth(lc1, node->resultRelations, lc2, node->fdwPrivLists)
> +           {
> +               if (lfirst_int(lc1) =3D=3D (int) rti)
> +               {
> +                   fdw_private =3D (List *) lfirst(lc2);
> +                   break;
> +               }
> +           }
>
> I think it's good to skip this for efficiency, when there are no
> pruned result relations.

Thanks for the review. v2 attached implements your suggestion: it
indexes node->fdwPrivLists directly with j when no result relations
were pruned, and only falls back to matching by range table index when
pruning appears to have dropped some.

> Other than that the patch looks good to me.
>
> I didn't know pg_rewrite's use of ModifyTableState, which was
> different than I expected.  Sorry for that.

No need to apologize at all. Tom's point about it was new to me too.

I will push v2 to REL_18 tomorrow barring objections.

--
Thanks, Amit Langote

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