Konstantin Knizhnik <k.knizhnik@postgrespro.ru> writes:
>> This example is lacking indexes on the child tables, which is
>> why the plan shown is about as good as you're going to get.
>> The contents of foo1 and foo2 have to be read in entirety in any
>> case, and sorting them separately is not a win compared to doing
>> a single sort.
> It is true, but not in case of FDW connected to remote host.
> In this case sending large volumes of data through network will be very
> inefficient.
If the FDW isn't providing a sorted path, there is no way to improve the
situation much. You can't just "push the LIMIT", you'd have to push
"ORDER BY ... LIMIT", which will mean that a sort has to happen anyway.
If the remote end can do a fast-start sort efficiently, it should report
that, and then a merge-append plan is just fine.
regards, tom lane
