Re: BUG #19484: Segmentation fault triggered by FDW

Amit-san,

On Thu, Jun 25, 2026 at 8:24 AM Amit Langote  wrote:
> On Thu, Jun 25, 2026 at 2:02 AM Etsuro Fujita  wrote:
> > This might be nitpicking, but:
> >
> > +           if (list_length(node->resultRelations) == mtstate->mt_nrels)
> > +               fdw_private = (List *) list_nth(node->fdwPrivLists, j);
> > +           else
> > +           {
> > +               Index       rti = resultRelInfo->ri_RangeTableIndex;
> > +               ListCell   *lc1;
> > +               ListCell   *lc2;
> > +
> > +               fdw_private = NIL;
> > +               forboth(lc1, node->resultRelations, lc2, node->fdwPrivLists)
> > +               {
> > +                   if (lfirst_int(lc1) == (int) rti)
> > +                   {
> > +                       fdw_private = (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!

Best regards,
Etsuro Fujita