Support triggers ON views.
 
This patch adds the SQL-standard concept OF an INSTEAD OF TRIGGER, which
IS fired instead OF performing a physical INSERT/UPDATE/DELETE.  The
TRIGGER FUNCTION IS passed the entire OLD AND/OR NEW ROWS OF the VIEW,
AND must figure OUT what TO do TO the underlying TABLES TO implement
the UPDATE.  So this feature can be used TO implement updatable views
USING TRIGGER programming STYLE rather than rule hacking.
 
IN passing, this patch corrects the names OF SOME COLUMNS IN the
information_schema.triggers VIEW.  It seems the SQL committee renamed
them somewhere BETWEEN SQL:99 AND SQL:2003.
 
Dean Rasheed, reviewed BY Bernd Helmle; SOME additional hacking BY me.

$ CREATE TABLE x AS SELECT i, 'value: ' || i AS ti FROM generate_series (1,10) i;
SELECT 10
 
$ SELECT * FROM x;
 i  |    ti
----+-----------
  1 | VALUE: 1
  2 | VALUE: 2
  3 | VALUE: 3
  4 | VALUE: 4
  5 | VALUE: 5
  6 | VALUE: 6
  7 | VALUE: 7
  8 | VALUE: 8
  9 | VALUE: 9
 10 | VALUE: 10
(10 ROWS)
 
$ CREATE VIEW test AS SELECT * FROM x;
CREATE VIEW

CREATE FUNCTION test_trigger() RETURNS TRIGGER AS $$
BEGIN
    IF TG_OP = 'INSERT' THEN
        raise notice 'INSERT trigger, NEW = [%]', NEW;
    ELSIF TG_OP = 'UPDATE' THEN
        raise notice 'UPDATE trigger, OLD = [%], NEW = [%]', OLD, NEW;
    ELSE
        raise notice 'DELETE trigger, OLD = [%]', OLD;
    END IF;
    RETURN NULL;
END;
$$ LANGUAGE plpgsql;
 
CREATE TRIGGER test_it INSTEAD OF INSERT OR UPDATE OR DELETE ON test FOR EACH ROW EXECUTE PROCEDURE test_trigger();

$ \d test 
      VIEW "public.test"
 COLUMN |  TYPE   | Modifiers 
--------+---------+-----------
 i      | INTEGER | 
 ti     | text    | 
Triggers:
    test_it INSTEAD OF INSERT OR DELETE OR UPDATE ON test FOR EACH ROW EXECUTE PROCEDURE test_trigger()

$ INSERT INTO test (ti) VALUES ( 'xx' );
NOTICE:  INSERT TRIGGER, NEW = [(,xx)]
INSERT 0 0
 
$ UPDATE test SET i = i + 123 WHERE i = 3;
NOTICE:  UPDATE TRIGGER, OLD = [(3,"value: 3")], NEW = [(126,"value: 3")]
UPDATE 0
 
$ DELETE FROM test WHERE i = 7;
NOTICE:  DELETE TRIGGER, OLD = [(7,"value: 7")]
DELETE 0