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In-Reply-To: 
 <CA+HiwqFd61sXgUWijKg8DfRQDtwYEGSdxuEPDb8rq22rxzxN_g@mail.gmail.com>
From: Etsuro Fujita <etsuro.fujita@gmail.com>
Date: Thu, 25 Jun 2026 02:02:41 +0900
Message-ID: 
 <CAPmGK15do2D1YJFR2VST6+8BnADyZbiiY1hk+HhMGgFyBkE70Q@mail.gmail.com>
Subject: Re: BUG #19484: Segmentation fault triggered by FDW
To: Amit Langote <amitlangote09@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>
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Amit-san,

On Wed, Jun 24, 2026 at 8:20=E2=80=AFPM Amit Langote <amitlangote09@gmail.c=
om> wrote:
> On Wed, Jun 24, 2026 at 6:15=E2=80=AFPM Etsuro Fujita <etsuro.fujita@gmai=
l.com> wrote:
> > +           /*
> > +            * node->fdwPrivLists is indexed by the original, pre-pruni=
ng
> > +            * result relation order and is parallel to node->resultRel=
ations.
> > +            * Initial pruning may have dropped earlier relations, so t=
he 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.

Thank you for doing that work!

This might be nitpicking, but:

+           if (list_length(node->resultRelations) =3D=3D mtstate->mt_nrels=
)
+               fdw_private =3D (List *) list_nth(node->fdwPrivLists, j);
+           else
+           {
+               Index       rti =3D resultRelInfo->ri_RangeTableIndex;
+               ListCell   *lc1;
+               ListCell   *lc2;
+
+               fdw_private =3D NIL;
+               forboth(lc1, node->resultRelations, lc2, node->fdwPrivLists=
)
+               {
+                   if (lfirst_int(lc1) =3D=3D (int) rti)
+                   {
+                       fdw_private =3D (List *) lfirst(lc2);
+                       break;
+                   }
+               }
+           }

I'd put the if-test outside of the outer loop to save cycles.

Other than that v2 looks good to me.

(The forboth loop actually causes an n-squared calculation, but it's
done only when pruning occurs, in which case the number of remaining
result relations would be reduced, so that wouldn't be a problem.)

Best regards,
Etsuro Fujita