The following is a remark about UPSERT in fdwhandler.sgml.
<command>INSERT</> with an <literal>ON CONFLICT</> clause does not
support specifying the conflict target, as remote constraints are not
locally known. This in turn implies that <literal>ON CONFLICT DO
UPDATE</> is not supported, since the specification is mandatory there.
ISTM that the sentence "as remote constraints are not locally known" is
somewhat confusing, because check constrains on remote tables can be
defined locally in 9.5. How about "unique constraints or exclusion
constraints on remote tables are not locally known"? Attached is a
patch for that.
Best regards,
Etsuro Fujita