На PostgreSQL 7.2.1:
...
# \d test_fk
Table "test_fk"
Column | Type | Modifiers
--------+----------+-----------
field | smallint |
Triggers: RI_ConstraintTrigger_3952505517
# ALTER TABLE test_fk DROP CONSTRAINT fk_constr RESTRICT;
ERROR: ALTER TABLE / DROP CONSTRAINT: fk_constr does not exist

На PostgreSQL 7.3.3:
...
# \d test_fk
Table "public.test_fk"
Column | Type | Modifiers
--------+----------+-----------
field | smallint |
Foreign Key constraints: fk_constr FOREIGN KEY (field) REFERENCES test(field) MATCH FULL ON UPDATE NO ACTION ON DELETE NO ACTION
# ALTER TABLE test_fk DROP CONSTRAINT fk_constr RESTRICT;
ALTER TABLE
# \d test_fk
Table "public.test_fk"
Column | Type | Modifiers
--------+----------+-----------
field | smallint |