Re: Varying results when using merge joins over postgres_fdw vs hash joins

Peter Geoghegan <pg(at)bowt(dot)ie> writes:
> I think that Corey describes a user hostile behavior. I feel that we
> should try to do better here.

It is that. I'm tempted to propose that we invent some kind of "unknown"
collation, which the planner would have to be taught to not equate to any
other column collation (not even other instances of "unknown"), and that
postgres_fdw's IMPORT ought to label remote columns with that collation
unless specifically told to do otherwise. Then it's on the user's head
if he tells us to do the wrong thing; but we won't produce incorrect
plans by default.

This is, of course, not at all a back-patchable fix.

regards, tom lane