DROP TABLE counters;
CREATE TABLE counters (
	id             integer,
	data_counters  integer DEFAULT 0
);

DROP TABLE data;
CREATE TABLE data (
	id           integer,
	counters_id  integer
);

/* RULES */
DROP RULE add_to_data;
CREATE RULE add_to_data AS ON insert TO data DO
	UPDATE counters SET data_counters = data_counters + 1 WHERE id = NEW.counters_id;

DROP RULE del_from_data;
CREATE RULE del_from_data AS ON delete TO data DO
	UPDATE counters SET data_counters = data_counters - 1 WHERE id = OLD.counters_id;

/* TESTS */
INSERT INTO counters (id) VALUES (1);

INSERT INTO data (id,counters_id) VALUES (1,1);
INSERT INTO data (id,counters_id) VALUES (2,1);
INSERT INTO data (id,counters_id) VALUES (3,1);
INSERT INTO data (id,counters_id) VALUES (4,1);

SELECT *,'data_counters should be == 4' AS expected FROM counters;

DELETE FROM data WHERE id = 1;
DELETE FROM data WHERE id = 2;
DELETE FROM data WHERE id = 3;
DELETE FROM data WHERE id = 4;

SELECT *,'data_counters should be == 0' AS expected FROM counters;