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