Re: BUG #15352: postgresql FDW error "ERROR: ORDER BY position 0 is not in select list"

On Tue, Aug 28, 2018 at 7:53 PM, Andrew Gierth
<andrew(at)tao11(dot)riddles(dot)org(dot)uk> wrote:
>>>>>> "Andrew" == Andrew Gierth <andrew(at)tao11(dot)riddles(dot)org(dot)uk> writes:
>
> Andrew> It seems obviously wrong that a constant pathkey with no actual
> Andrew> reference to the foreign table should be being pushed down, so
> Andrew> so far I suspect that get_useful_pathkeys_for_relation isn't
> Andrew> being selective enough about what is "useful". In this context
> Andrew> I find it suspicious that find_em_expr_for_rel will return an
> Andrew> expr with no vars as being "for" every rel, since it's just
> Andrew> looking for a subset.

Sorry for replying late. I am not able to understand why it's wrong to
push a constant or for the matter any shippable expression which
doesn't refer to the foreign table/s (for that matter any tables)
under consideration down to the foreign server. The context in the
original mail doesn't help. I haven't checked the original thread on
bugs mailing list. I agree that ordering by such an expression is
useless, but if we are getting that done from a foreign server, what's
the harm? But by not doing it we might be loosing some optimization
since postgres_fdw pushes all or none of pathkeys.

--
Best Wishes,
Ashutosh Bapat
EnterpriseDB Corporation
The Postgres Database Company