hi.
about this issue,
last email in 2012 (https://postgr.es/m/8967.1353167301@sss.pgh.pa.us)
"""
Even if it happens to be trivial in the current patch, it's an added
functional requirement that we might later regret having cavalierly
signed up for. And, as noted upthread, relations that support only
one direction of COPY don't exist at the moment; that would be adding
an asymmetry that we might later regret, too.
regards, tom lane
"""
but now we have numerous COPY options that work solely in a single
direction of COPY.
I think now we can make some kind of relation (pg_class.relkind) that
only works in one direction of COPY.