Fix interaction of foreign tuple routing with remote triggers.

Without these fixes, changes to the inserted tuple made by remote
triggers are ignored when building local RETURNING tuples.

In the core code, call ExecInitRoutingInfo at a later point from
within ExecInitPartitionInfo so that the FDW callback gets invoked
after the returning list has been built.  But move CheckValidResultRel
out of ExecInitRoutingInfo so that it can happen at an earlier stage.

In postgres_fdw, refactor assorted deparsing functions to work with
the RTE rather than the PlannerInfo, which saves us having to
construct a fake PlannerInfo in cases where we don't have a real one.
Then, we can pass down a constructed RTE that yields the correct
deparse result when no real one exists.  Unfortunately, this
necessitates a hack that understands how the core code manages RT
indexes for update tuple routing, which is ugly, but we don't have a
better idea right now.

Original report, analysis, and patch by Etsuro Fujita.  Heavily
refactored by me.  Then worked over some more by Amit Langote.

Discussion: http://postgr.es/m/5AD4882B.10002@lab.ntt.co.jp

contrib/postgres_fdw/sql/postgres_fdw.sql

@@ -1804,6 +1804,31 @@ insert into itrtest values (1, 'bar') on conflict (a) do update set b = excluded
 
 select tableoid::regclass, * FROM itrtest;
 
+delete from itrtest;
+
+drop index loct1_idx;
+
+-- Test that remote triggers work with insert tuple routing
+create function br_insert_trigfunc() returns trigger as $$
+begin
+	new.b := new.b || ' triggered !';
+	return new;
+end
+$$ language plpgsql;
+create trigger loct1_br_insert_trigger before insert on loct1
+	for each row execute procedure br_insert_trigfunc();
+create trigger loct2_br_insert_trigger before insert on loct2
+	for each row execute procedure br_insert_trigfunc();
+
+-- The new values are concatenated with ' triggered !'
+insert into itrtest values (1, 'foo') returning *;
+insert into itrtest values (2, 'qux') returning *;
+insert into itrtest values (1, 'test1'), (2, 'test2') returning *;
+with result as (insert into itrtest values (1, 'test1'), (2, 'test2') returning *) select * from result;
+
+drop trigger loct1_br_insert_trigger on loct1;
+drop trigger loct2_br_insert_trigger on loct2;
+
 drop table itrtest;
 drop table loct1;
 drop table loct2;
@@ -1836,6 +1861,30 @@ select tableoid::regclass, * FROM locp;
 -- The executor should not let unexercised FDWs shut down
 update utrtest set a = 1 where b = 'foo';
 
+-- Test that remote triggers work with update tuple routing
+create trigger loct_br_insert_trigger before insert on loct
+	for each row execute procedure br_insert_trigfunc();
+
+delete from utrtest;
+insert into utrtest values (2, 'qux');
+
+-- Check case where the foreign partition is a subplan target rel
+explain (verbose, costs off)
+update utrtest set a = 1 where a = 1 or a = 2 returning *;
+-- The new values are concatenated with ' triggered !'
+update utrtest set a = 1 where a = 1 or a = 2 returning *;
+
+delete from utrtest;
+insert into utrtest values (2, 'qux');
+
+-- Check case where the foreign partition isn't a subplan target rel
+explain (verbose, costs off)
+update utrtest set a = 1 where a = 2 returning *;
+-- The new values are concatenated with ' triggered !'
+update utrtest set a = 1 where a = 2 returning *;
+
+drop trigger loct_br_insert_trigger on loct;
+
 drop table utrtest;
 drop table loct;