>The actual implementation of this example involves a much more complicated scenario, but I think this expresses the heart of my question.
>
>Calculate sum(b) for a > 10 and a < 15 to m_sum_b
>
>Will execution be speeded up if a is indexed?

As other have said if there is an INDEX ON A your code will optimize. You can also use BETWEEN(). Calculate sum(b) FOR BETWEEN(a,10,15) to m_sum_b