Re: Foreign join pushdown vs EvalPlanQual

> -----Original Message-----
> From: pgsql-hackers-owner(at)postgresql(dot)org
> [mailto:pgsql-hackers-owner(at)postgresql(dot)org] On Behalf Of Robert Haas
> Sent: Friday, November 06, 2015 9:40 PM
> To: Kaigai Kouhei(海外 浩平)
> Cc: Etsuro Fujita; Tom Lane; Kyotaro HORIGUCHI; pgsql-hackers(at)postgresql(dot)org;
> Shigeru Hanada
> Subject: Re: [HACKERS] Foreign join pushdown vs EvalPlanQual
>
> On Tue, Nov 3, 2015 at 8:15 AM, Kouhei Kaigai <kaigai(at)ak(dot)jp(dot)nec(dot)com> wrote:
> > A challenge is that junk wholerow references on behalf of ROW_MARK_COPY
> > are injected by preprocess_targetlist(). It is earlier than the main path
> > consideration by query_planner(), thus, it is not predictable how remote
> > query shall be executed at this point.
>
> Oh, dear. That seems like a rather serious problem for my approach.
>
> > If ROW_MARK_COPY, base tuple image is fetched using this junk attribute.
> > So, here is two options if we allow to put joined tuple on either of
> > es_epqTuple[].
>
> Neither of these sounds viable to me.
>
> I'm inclined to go back to something like what you proposed here:
>
Good :-)

> http://www.postgresql.org/message-id/9A28C8860F777E439AA12E8AEA7694F80114B89
> D(at)BPXM15GP(dot)gisp(dot)nec(dot)co(dot)jp
>
This patch needs to be rebased.
One thing different from the latest version is fdw_recheck_quals of
ForeignScan was added. So, ...

(1) Principle is that FDW driver knows what qualifiers were pushed down
and how does it kept in the private field. So, fdw_recheck_quals is
redundant and to be reverted.

(2) Even though the principle is as described in (1), however,
wired logic in ForeignRecheck() and fdw_recheck_quals are useful
default for most of FDW drivers. So, it shall be kept and valid
only if RecheckForeignScan callback is not defined.

Which is better approach for the v3 patch?
My preference is (1), because fdw_recheck_quals is a new feature,
thus, FDW driver has to be adjusted in v9.5 more or less, even if
it already supports qualifier push-down.
In general, interface becomes more graceful to stick its principle.

Thanks,
--
NEC Business Creation Division / PG-Strom Project
KaiGai Kohei <kaigai(at)ak(dot)jp(dot)nec(dot)com>