Help with pl/pgsql, triggers, and foreign keys

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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 TABLE task (
project->     task_id             INT PRIMARY KEY,
project->     task_pid            INT
project-> );
NOTICE:  CREATE TABLE/PRIMARY KEY will create implicit index 'task_pkey' for table 'task'
CREATE
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();
CREATE
project=> insert into task values (1, null);
INSERT 27855 1
project=> insert into task values (2, null);
INSERT 27856 1
project=> insert into task values (3, 1);
ERROR:  unexpected SELECT query in exec_stmt_execsql()

roland
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Roland B. Roberts, PhD                  Custom Software Solutions
roberts@panix.com                      76-15 113th Street, Apt 3B
rbroberts@acm.org                          Forest Hills, NY 11375

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