adding foreign key constraint locks up table

I have a table "aaa" which is not very big. It has less than 10'000
rows. However read operations on this table is very frequent.

Whenever I try to create a new table "bbb" with foreign key pointing
to "aaa". The operation locks, and reading "aaa" is not possible. The
query also never seems to finish.

ALTER TABLE "bbb" ADD CONSTRAINT "topic_id_refs_id_3942a46c6ab2c0b4"
FOREIGN KEY ("topic_id") REFERENCES "aaa" ("id") DEFERRABLE INITIALLY
DEFERRED;

The current workaround is to create any new table at off-peak hours,
e.g. midnight after restarting the db.

I would like to know if there's any proper solution of this. Is this
an issue affecting all relational databases? My db is PostgreSQL 8.3.