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

Peter Eisentraut <peter(dot)eisentraut(at)2ndquadrant(dot)com> writes:
> On 9/20/17 12:06, Tom Lane wrote:
>> 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.

> OID 0 might already work that way, depending on the details.

No, OID 0 means "column is not collatable". I'm pretty sure there are
some asserts that will trip if we use that collation OID for a column of a
collatable data type --- and even if there are not, I think conflating the
two cases would be a bad idea.

regards, tom lane