Problem with transition tables on partitioned tables with foreign-table partitions

Hi,

While working on something else, I noticed that while we disallow
transition tables on foreign tables, we allow transition tables on
partitioned tables with foreign-table partitions, which produces
incorrect results. Here is an example using postgres_fdw:

create table parent (a text, b int) partition by list (a);
create table loct (a text, b int);
create foreign table child (a text, b int)
server loopback options (table_name 'loct');
alter table parent attach partition child for values in ('AAA');

create function dump_insert() returns trigger language plpgsql as
$$
begin
raise notice 'trigger = %, new table = %',
TG_NAME,
(select string_agg(new_table::text, ', ' order by a)
from new_table);
return null;
end;
$$;
create trigger parent_insert_trig
after insert on parent referencing new table as new_table
for each statement execute procedure dump_insert();

create function intercept_insert() returns trigger language plpgsql as
$$
begin
new.b = new.b + 1000;
return new;
end;
$$;
create trigger intercept_insert_loct
before insert on loct
for each row execute procedure intercept_insert();

insert into parent values ('AAA', 42);
NOTICE: trigger = parent_insert_trig, new table = (AAA,42)
INSERT 0 1

The trigger shows the original tuple created by the core, not the
actual tuple inserted into the foreign-table partition, as
postgres_fdw does not collect the actual tuple, of course!

UPDATE/DELETE also produce incorrect results:

create function dump_update() returns trigger language plpgsql as
$$
begin
raise notice 'trigger = %, old table = %, new table = %',
TG_NAME,
(select string_agg(old_table::text, ', ' order by a)
from old_table),
(select string_agg(new_table::text, ', ' order by a)
from new_table);
return null;
end;
$$;
create trigger parent_update_trig
after update on parent referencing old table as old_table new table
as new_table
for each statement execute procedure dump_update();

update parent set b = b + 1;
NOTICE: trigger = parent_update_trig, old table = <NULL>, new table = <NULL>
UPDATE 1

create function dump_delete() returns trigger language plpgsql as
$$
begin
raise notice 'trigger = %, old table = %',
TG_NAME,
(select string_agg(old_table::text, ', ' order by a)
from old_table);
return null;
end;
$$;
create trigger parent_delete_trig
after delete on parent referencing old table as old_table
for each statement execute procedure dump_delete();

delete from parent;
NOTICE: trigger = parent_delete_trig, old table = <NULL>
DELETE 1

In both cases the triggers fail to show transition tuples. The cause
of this is that postgres_fdw mistakenly performs direct modify for
UPDATE/DELETE on the partition, which skips
ExecARUpdateTriggers()/ExecARDeleteTriggers() entirely.

To fix, I think we could disallow creating transition-table triggers
on such partitioned tables, but I think that that is too restrictive
because some users might have been using such triggers, avoiding this
problem by e.g., modifying only plain-table partitions. So I would
like to propose to fix this by the following: 1) disable using direct
modify to modify foreign-table partitions if there are any
transition-table triggers on the partitioned table, and then 2) throw
an error in ExecARInsertTriggers()/ExecARUpdateTriggers()/ExecARDeleteTriggers()
if they collects transition tuple(s) from a foreign-table partition.

Attached is a WIP patch for that.

Best regards,
Etsuro Fujita