Re: BUG #19484: Segmentation fault triggered by FDW

Amit-san,

On Wed, Jun 24, 2026 at 8:20 PM Amit Langote  wrote:
> On Wed, Jun 24, 2026 at 6:15 PM Etsuro Fujita  wrote:
> > +           /*
> > +            * node->fdwPrivLists is indexed by the original, pre-pruning
> > +            * result relation order and is parallel to node->resultRelations.
> > +            * 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) == (int) rti)
> > +               {
> > +                   fdw_private = (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) == 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.

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