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From: Amit Langote <amitlangote09@gmail.com>
Date: Thu, 25 Jun 2026 20:47:22 +0900
Message-ID: 
 <CA+HiwqE_oxre_URagYTSx1fpG_+ZVaY8VkxQtEyceHxYZGLcjQ@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>
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Precedence: bulk

Fujita-san,

On Thu, Jun 25, 2026 at 3:15=E2=80=AFPM Etsuro Fujita <etsuro.fujita@gmail.=
com> wrote:
> On Thu, Jun 25, 2026 at 8:24=E2=80=AFAM Amit Langote <amitlangote09@gmail=
.com> wrote:
> > On Thu, Jun 25, 2026 at 2:02=E2=80=AFAM Etsuro Fujita <etsuro.fujita@gm=
ail.com> wrote:
> > > 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->fdwPri=
vLists)
> > > +               {
> > > +                   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.
> >
> > Right, it's loop-invariant. v3 attached computes a boolean
> > (nopruning), like labeltargets, once before the loop and uses it
> > inside.
> >
> > > 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.)
> >
> > Right, though strictly the inner forboth scans node->resultRelations,
> > which pruning leaves at its original length, so it's the original
> > relation count that bounds the scan rather than the reduced one.
> > Either way it's EXPLAIN-only with small counts, so it's not a concern.
>
> That's right.
>
> The v3 patch looks good to me.  Thanks for updating the patch!

Pushed, thanks for checking.

--=20
Thanks, Amit Langote