pg_locks.relation question

Hi,

session one:

locks=# begin;
BEGIN
locks=# alter table test add column b text;
ALTER TABLE
locks=#

Session 2 querying pg_locks for the PID from above:

locks=# select locktype,relation::regclass,mode from pg_locks where pid = 2026 and locktype = 'relation';
locktype | relation | mode
----------+----------+---------------------
relation | test | AccessExclusiveLock
relation | 17728 | AccessExclusiveLock
relation | 17726 | ShareLock
(3 rows)

What are these 17728 and 17726 OIDs? Nothing else is happening except these two sessions.

Thanks in advance
Daniel