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

>>>>> "Andrew" == Andrew Gierth <andrew@tao11.riddles.org.uk> writes:

 Andrew> So what's happening here is that there's an equivalence class
 Andrew> containing members
 Andrew> "greatest(pg_catalog.pg_stat_user_tables.idx_scan,
 Andrew> public.pg_stat_user_tables.idx_scan)" and "0" (as an integer
 Andrew> constant), and somehow a pathkey for this eclass is becoming
 Andrew> attached to the query going to the remote for statistics
 Andrew> purposes.

 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.

So this looks to me like an oversight in aa09cd242 (CCing rhaas and
Ashutosh accordingly), which changed find_em_expr_for_rel from using
bms_equal to bms_is_subset without considering the degenerate case of
members with no relids at all. I propose to simply add a !bms_is_empty
condition there; anyone have any better idea?

-- 
Andrew (irc:RhodiumToad)