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
NULLTHEN           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