On Wed, Aug 29, 2018 at 4:32 PM, Andrew Gierth
<andrew@tao11.riddles.org.uk> wrote:
>>>>>> "Ashutosh" == Ashutosh Bapat <ashutosh.bapat@enterprisedb.com> 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.
>
> Ashutosh> Sorry for replying late. I am not able to understand why it's
> Ashutosh> wrong to push a constant or for the matter any shippable
> Ashutosh> expression which doesn't refer to the foreign table/s (for
> Ashutosh> that matter any tables) under consideration down to the
> Ashutosh> foreign server.
>
> Well, it's certainly pointless.
>
> But the failure in this case is specifically about pushing down an
> _integer_ constant, because the deparse code for pushing down an ORDER
> BY does not understand that integer literals in ORDER BY clauses are a
> special case.
Deparser needs to be fixed then, irrespective of whether or not we fix
the costant pathkey problem.
>
> Ashutosh> The context in the original mail doesn't help. I haven't
> Ashutosh> checked the original thread on bugs mailing list. I agree
> Ashutosh> that ordering by such an expression is useless, but if we are
> Ashutosh> getting that done from a foreign server, what's the harm? But
> Ashutosh> by not doing it we might be loosing some optimization since
> Ashutosh> postgres_fdw pushes all or none of pathkeys.
>
> I'm pretty sure that constant (hence redundant) clauses have been
> removed from pathkeys before postgres_fdw will see them. The problem
> only occurs because postgres_fdw tries inventing _new_ pathkeys for
> possible orderings from eclasses (in order to try for mergejoin
> opportunities) in addition to using the requested pathkeys, and it's
> clearly pointless to do that with constants.
>
Yes, I forgot about that. But even in that case, we should consider
the case when the constant pathkey is just one in the bunch and we are
trying to push the whole bunch.
--
Best Wishes,
Ashutosh Bapat
EnterpriseDB Corporation
The Postgres Database Company