Re: 9.5 release notes may need ON CONFLICT DO NOTHING compatibility notice for FDW authors

There is no support for ON CONFLICT DO UPDATE with postgres_fdw, but
that's really only because an inference specification (or explicitly
named constraint) is always required for DO UPDATE. The deparsing
support actually added will have deparsing add "ON CONFLICT DO
NOTHING" for the SQL generated for execution on foreign servers if the
original statement had that exact, unadorned ON CONFLICT clause. As
things stand, every other possible ON CONFLICT clause will throw an
error in some way before the FDW is consulted at all, so FDW authors
need not concern themselves with those other cases (unless perhaps we
allow ON CONFLICT DO UPDATE to not require an inference specification
in a last minute behavioral tweak, as suggested by Simon Riggs, making
ON CONFLICT DO UPDATE support by foreign data wrappers a possibility
that must be considered).