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

> > One thing tricky is "peusdo projection" which is done by
> > deparseProjectionSql for whole-row reference. This is done by put the
> > query string in FROM subquery and add whole-row reference in outer
> > SELECT clause. This is done by ExecProjection in 9.4 and older, but
> > we need to do this in postgres_fdw because ExecProjection is not
> > called any more.
>
> What commit changed this?
>
It seems to me the nature of whole-row reference, not a flaw of
ExecProject().

In case of base relation scan, whole-row reference is transformed
to ExecEvalWholeRowVar, then executed during ExecProject().
It constructs a record datum according to the TupleDesc of the
relation being in scan.
On the other hands, foreign-join also looks like a scan on relation
that is result set of remote join, its record type is defined in
the fdw_scan_tlist.
However, it may contain values come from multiple relations,
so it is not intended behavior if whole-row reference constructs
a record datum that contains all the attributes in the result-
set. In this context, whole-row reference shall contain all the
attributes of the "base" relation.
Only FDW driver can know, and ensure all the attributes to
construct whole-row reference are fetched from remote side.

Hanada-san, could you correct me, if I misunderstood above your
explanation?

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