> In a setting where every operator has exactly one output, when to run an operator to produce some output is obvious: when your consumer needs it. This becomes, at the very least, messier with multiple outputs, since “requests for rows” and “computations to produce rows” are no longer one-to-one.

Don't push-based systems have the same issue for inputs? If you have multiple inputs then you may be handled one of the inputs without the other. The classic example with iterators is a `zip` operator, though maybe this is probably not so common in database queries.