> Is there anyway to know if a DELETE will violate an FK
> without actually trying it?
I don't know what you mean by 'without trying it', but does the
following answer your question?
CREATE TABLE a (id integer PRIMARY KEY);
CREATE TABLE b (id integer PRIMARY KEY,
a_id integer NOT NULL CONSTRAINT b_fkey REFERENCES a(id));
INSERT INTO a (id) VALUES (1);
INSERT INTO b (id, a_id) VALUES (42, 1);
DELETE FROM a WHERE id=1;
ERROR: update or delete on table "a" violates foreign key constraint
"b_fkey" on table "b"
DETAIL: Key (id)=(1) is still referenced from table "b".
Yours,
Laurenz Albe