One way to do this is with a trigger!
CREATE TABLE computed(
one SERIAL,
two INT NOT NULL
);
CREATE OR REPLACE FUNCTION computed_two_trg()
RETURNS trigger
LANGUAGE plpgsql
SECURITY DEFINER
AS $BODY$
BEGIN
NEW.two = NEW.one * 2;
RETURN NEW;
END
$BODY$;
CREATE TRIGGER computed_500
BEFORE INSERT OR UPDATE
ON computed
FOR EACH ROW
EXECUTE PROCEDURE computed_two_trg();
The trigger is fired before the row is updated or inserted. It changes the field that we want to compute of NEW record and then it returns that record.