Re: [HACKERS] Help with pl/pgsql, triggers, and foreign keys

Roland Roberts wrote:
>
> The Postgres guide says that foreign keys can be partially emulated
> via triggers. Just how "partial" is this. I can't seem to get the
> following to work. Would it work if I wrote it in C? Would I need to
> open a second connection to the database? Would it work if my second
> key was really in another table?
>
> project=> CREATE FUNCTION check_task_pid () RETURNS OPAQUE AS '
> project'> BEGIN
> project'> IF NEW.task_pid IS NOT NULL THEN
> project'> SELECT task_id FROM task WHERE task_id = NEW.task_pid;
> project'> IF NOT FOUND THEN
> project'> RAISE EXCEPTION ''FOREIGN KEY constraint task_pid not found'';
> project'> END IF;
> project'> END IF;
> project'> RETURN NEW;
> project'> END;
> project'> ' LANGUAGE 'plpgsql';
> CREATE
> project=> CREATE TRIGGER iu_btrig_check_task_pid BEFORE INSERT OR UPDATE ON task
> project-> FOR EACH ROW EXECUTE PROCEDURE check_task_pid();

At least in 6.5.2, you can definitely implement referential integrity
(RI) via pl/pgsql. As someone noted earlier, RI is to be released in
7.0, but I suspect it will take a subsequent release or two to
stabilize before it's fit for consumption by the more conservative
reliability-focused users among us...

As for your failing SELECT query, the following tweak to your function
makes it work as expected:

CREATE FUNCTION check_task_pid () RETURNS OPAQUE AS '
DECLARE
tmp RECORD;
BEGIN
IF NEW.task_pid IS NOT NULL THEN
SELECT INTO tmp task_id FROM task WHERE task_id =
NEW.task_pid;
IF NOT FOUND THEN
RAISE EXCEPTION ''FOREIGN KEY constraint task_pid not
found'';
END IF;
END IF;
RETURN NEW;
END;
' LANGUAGE 'plpgsql';

Cheers,
Ed Loehr