Re: Optimization for updating foreign tables in Postgres FDW

Hi,

While re-reviewing the fix, I noticed that since PQcancel we added to
pgfdw_xact_callback to cancel a DML pushdown query isn't followed by a
ROLLBACK, the connection to the remote server will be discarded at the
end of the while loop in that function, which will cause a FATAL error
of "connection to client lost". Probably, that was proposed by me in
the first version of the patch, but I don't think that's a good idea.
Shouldn't we execute ROLLBACK after that PQcancel?

Another thing I noticed is, ISTM that we miss the case where DML
pushdown queries are performed in subtransactions. I think cancellation
logic would also need to be added to pgfdw_subxact_callback.

Comments are welcome!

Best regards,
Etsuro Fujita