DB alert on values change
You have multiple choices here. 


writing a crontab that will check every minutes your data (that's what you propose). 
using NOTIFY/LISTEN with trigger like this :



CREATE OR REPLACE FUNCTION f_check_data_value()
  RETURNS TRIGGER
LANGUAGE plpgsql
AS $$
BEGIN
  SELECT pg_notify('data', NEW.value);
END;
$$;

CREATE TRIGGER check_data_values_trigger
AFTER INSERT OR UPDATE
  ON mytable
FOR EACH ROW EXECUTE PROCEDURE f_check_data_value();


You'll then have to write a listener (in PHP if it's what you are using). I haven't used php inn a while but the php.net documentation is a good place to start I think : http://php.net/manual/fr/function.pg-get-notify.php