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

Hi Fujita-san,

On Tue, Jul 1, 2025 at 11:55 AM Etsuro Fujita <etsuro(dot)fujita(at)gmail(dot)com> wrote:
>
> 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!

Maybe I'm missing something, but given that the intercept_insert()
function is applied during the "remote" operation, isn't it expected
that the parent table's trigger for a "local" operation shows the
original tuple?

> 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.

Yes, that seems problematic.

> 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.

+1

> 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.

Is (2) intended to catch cases that occur during a foreign insert and
foreign/non-direct update/delete?

--
Thanks, Amit Langote