Allow batch insertion during COPY into a foreign table.

Commit 3d956d956 allowed the COPY, but it's done by inserting individual
rows to the foreign table, so it can be inefficient due to the overhead
caused by each round-trip to the foreign server.  To improve performance
of the COPY in such a case, this patch allows batch insertion, by
extending the multi-insert machinery in CopyFrom() to the foreign-table
case so that we insert multiple rows to the foreign table at once using
the FDW callback routine added by commit b663a4136.  This patch also
allows this for postgres_fdw.  It is enabled by the "batch_size" option
added by commit b663a4136, which is disabled by default.

When doing batch insertion, we update progress of the COPY command after
performing the FDW callback routine, to count rows not suppressed by the
FDW as well as a BEFORE ROW INSERT trigger.  For consistency, this patch
changes the timing of updating it for plain tables: previously, we
updated it immediately after adding each row to the multi-insert buffer,
but we do so only after writing the rows stored in the buffer out to the
table using table_multi_insert(), which I think would be consistent even
with non-batching mode, because in that mode we update it after writing
each row out to the table using table_tuple_insert().

Andrey Lepikhov, heavily revised by me, with review from Ian Barwick,
Andrey Lepikhov, and Zhihong Yu.

Discussion: https://postgr.es/m/bc489202-9855-7550-d64c-ad2d83c24867%40postgrespro.ru

contrib/postgres_fdw/sql/postgres_fdw.sql

@@ -2373,6 +2373,28 @@ copy remp1 from stdin;
 
 select tableoid::regclass, * FROM remp1;
 
+delete from ctrtest;
+
+-- Test copy tuple routing with the batch_size option enabled
+alter server loopback options (add batch_size '2');
+
+copy ctrtest from stdin;
+1	foo
+1	bar
+2	baz
+2	qux
+1	test1
+2	test2
+\.
+
+select tableoid::regclass, * FROM ctrtest;
+select tableoid::regclass, * FROM remp1;
+select tableoid::regclass, * FROM remp2;
+
+delete from ctrtest;
+
+alter server loopback options (drop batch_size);
+
 drop table ctrtest;
 drop table loct1;
 drop table loct2;
@@ -2527,6 +2549,86 @@ select * from rem3;
 drop foreign table rem3;
 drop table loc3;
 
+-- Test COPY FROM with the batch_size option enabled
+alter server loopback options (add batch_size '2');
+
+-- Test basic functionality
+copy rem2 from stdin;
+1	foo
+2	bar
+3	baz
+\.
+select * from rem2;
+
+delete from rem2;
+
+-- Test check constraints
+alter table loc2 add constraint loc2_f1positive check (f1 >= 0);
+alter foreign table rem2 add constraint rem2_f1positive check (f1 >= 0);
+
+-- check constraint is enforced on the remote side, not locally
+copy rem2 from stdin;
+1	foo
+2	bar
+3	baz
+\.
+copy rem2 from stdin; -- ERROR
+-1	xyzzy
+\.
+select * from rem2;
+
+alter foreign table rem2 drop constraint rem2_f1positive;
+alter table loc2 drop constraint loc2_f1positive;
+
+delete from rem2;
+
+-- Test remote triggers
+create trigger trig_row_before_insert before insert on loc2
+	for each row execute procedure trig_row_before_insupdate();
+
+-- The new values are concatenated with ' triggered !'
+copy rem2 from stdin;
+1	foo
+2	bar
+3	baz
+\.
+select * from rem2;
+
+drop trigger trig_row_before_insert on loc2;
+
+delete from rem2;
+
+create trigger trig_null before insert on loc2
+	for each row execute procedure trig_null();
+
+-- Nothing happens
+copy rem2 from stdin;
+1	foo
+2	bar
+3	baz
+\.
+select * from rem2;
+
+drop trigger trig_null on loc2;
+
+delete from rem2;
+
+-- Check with zero-column foreign table; batch insert will be disabled
+alter table loc2 drop column f1;
+alter table loc2 drop column f2;
+alter table rem2 drop column f1;
+alter table rem2 drop column f2;
+copy rem2 from stdin;
+
+
+
+\.
+select * from rem2;
+
+delete from rem2;
+
+alter server loopback options (drop batch_size);
+
 -- ===================================================================
 -- test for TRUNCATE
 -- ===================================================================