(depesz@[local]:5840) 17:56:46 [depesz]
# create table comm (forumid int4);
CREATE TABLE
Time: 14.931 ms
(depesz@[local]:5840) 17:56:54 [depesz]
# create table forum (id int4);
CREATE TABLE
Time: 22.735 ms
(depesz@[local]:5840) 17:56:59 [depesz]
# insert into comm (forumid) values (1),(2);
INSERT 0 2
Time: 11.470 ms
(depesz@[local]:5840) 17:57:23 [depesz]
# insert into forum (id) values (1),(2),(3),(4);
INSERT 0 4
Time: 7.313 ms
(depesz@[local]:5840) 17:57:32 [depesz]
# select id from forum where id not in (Select forumid from comm);
 id
----
  3
  4
(2 rows)

Time: 0.811 ms
(depesz@[local]:5840) 17:57:41 [depesz]
# insert into comm (forumid) values (NULL);
INSERT 0 1
Time: 5.063 ms
(depesz@[local]:5840) 17:57:49 [depesz]
# select id from forum where id not in (Select forumid from comm);
 id
----
(0 rows)