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

On Mon, Feb 8, 2016 at 5:45 AM, Etsuro Fujita
<fujita(dot)etsuro(at)lab(dot)ntt(dot)co(dot)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
The Enterprise PostgreSQL Company