Re: [POC] Fast COPY FROM command for the table with foreign partitions

This patch currently looks very ready for use. And I'm taking a close
look at the error reporting. Here we have difference in behavior of
local and foreign table:

regression test in postgres_fdw.sql:
copy rem2 from stdin;
-1 xyzzy
\.

reports error (1):
=================
ERROR: new row for relation "loc2" violates check constraint...
DETAIL: Failing row contains (-1, xyzzy).
CONTEXT: COPY loc2, line 1: "-1 xyzzy"
remote SQL command: COPY public.loc2(f1, f2) FROM STDIN
COPY rem2, line 2

But local COPY into loc2 reports another error (2):
===================================================
copy loc2 from stdin;
ERROR: new row for relation "loc2" violates check constraint...
DETAIL: Failing row contains (-1, xyzzy).
CONTEXT: COPY loc2, line 1: "-1 xyzzy"

Report (2) is shorter and more specific.
Report (1) contains meaningless information.

Maybe we need to improve error report? For example like this:
ERROR: Failed COPY into foreign table "rem2":
new row for relation "loc2" violates check constraint...
DETAIL: Failing row contains (-1, xyzzy).
remote SQL command: COPY public.loc2(f1, f2) FROM STDIN
COPY rem2, line 1

The problem here is that we run into an error after the COPY FROM
command completes. And we need to translate lineno from foreign server
to lineno of overall COPY command.

--
regards,
Andrey Lepikhov
Postgres Professional