CREATE OR REPLACE FUNCTION insert_quantity()
  RETURNS trigger AS
$$
BEGIN
  IF NOT EXISTS (SELECT 1 FROM product_quantities where id=NEW.id) THEN
    INSERT INTO product_quantities(id,quantity) VALUES (NEW.id,0);
  END IF;
  RETURN NEW;
END;
$$
LANGUAGE plpgsql;

DROP TRIGGER IF EXISTS product_quantity_trigger ON public.products;
CREATE TRIGGER product_quantity_trigger
   AFTER INSERT OR UPDATE ON products
   FOR EACH ROW
   EXECUTE PROCEDURE insert_quantity();