DROP SEQUENCE next_id;
DROP TABLE ids;

CREATE SEQUENCE next_id START -2 MINVALUE -2;

CREATE TABLE ids (
	id		int4,
	idesc		text
);

CREATE TRIGGER ids_nextid 
	BEFORE INSERT OR UPDATE ON ids
	FOR EACH ROW 
	EXECUTE PROCEDURE autoinc (id, next_id);

INSERT INTO ids VALUES (0, 'first (-2 ?)');
INSERT INTO ids VALUES (null, 'second (-1 ?)');
INSERT INTO ids(idesc) VALUES ('third (1 ?!)');

SELECT * FROM ids;

UPDATE ids SET id = null, idesc = 'first: -2 --> 2' 
	WHERE idesc = 'first (-2 ?)';
UPDATE ids SET id = 0, idesc = 'second: -1 --> 3' 
	WHERE id = -1;
UPDATE ids SET id = 4, idesc = 'third: 1 --> 4' 
	WHERE id = 1;

SELECT * FROM ids;

SELECT 'Wasn''t it 4 ?' as nextval, nextval ('next_id') as value;

insert into ids (idesc) select textcat (idesc, '. Copy.') from ids;

SELECT * FROM ids;