Re: postgres_fdw join pushdown (was Re: Custom/Foreign-Join-APIs)

On Mon, Feb 8, 2016 at 5:45 AM, Etsuro Fujita
<fujita.etsuro@lab.ntt.co.jp> wrote:
> Maybe my explanation was not correct, but I'm saying that the targertlist of
> the above outer_plan should be set to the fdw_scan_tlist, to avoid
> misbehavior.

Yeah, I think you're right.  So in this hunk:

+       if (foreignrel->reloptkind == RELOPT_JOINREL)
+       {
+               /* For a join relation, get the conditions from
fdw_private structure */
+               remote_conds = fpinfo->remote_conds;
+               local_exprs = fpinfo->local_conds;
+
+               /* Build the list of columns to be fetched from the
foreign server. */
+               fdw_scan_tlist = build_tlist_to_deparse(foreignrel);
+       }

I think we should also be doing outer_plan->targetlist =
fdw_scan_tlist in this block, with a comment like "Ensure that the
outer plan produces the a tuple whose descriptor matches our scan
tuple slot.  This is safe because all scans and joins support
projection, so we never need to insert a Result node."   It would
probably be good to Assert(outer_plan != NULL) before doing the
assignment, too, just as a guard against future bugs.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
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