postgres_fdw prefers fast plans

Hi.

There's a long-standing issue with postgres_fdw - as it uses cursors, it 
prefers plans, optimized for fetching first rows. In bad scenarios this 
leads to suboptimal choice of join methods (e.g. choosing nest loop over 
hash join) on remote side. I've crafted WIP patch, which tries to fix 
this issue. It adds FETCH ALL cursors and uses them in postgres_fdw. 
What do you think? Should we go in this direction?

I've looked at attempt to avoid cursors with postgres_fdw [1], but it 
seems to avoid dealing with async foreign plans (and looks more 
intrusive).


1) 
https://www.postgresql.org/message-id/flat/CA%2BFpmFcmO5ctjYgQxSomJC%3DmCugqPo%2B51Le2wdxX0kWxjvBBig%40mail.gmail.com

-- 
Best regards,
Alexander Pyhalov,
Postgres Professional