with verbs of motion по has the notion of setting out, initiating the activity.
Он шел на почту. He was going to the post office.
Он пошел на почту. He's gone off to the post office.

In others, it adds the notion of 'a little bit' -- давай поем! Let's get a bite to eat! . Где можно что-то почитать про Informix ? Where can I read some on Informix?

I am sure some reference book will have generalized meanings associated with various prefixes, but they don't always have to have meaning. Look at на 'on'. In perfective of писать, it has absolutely no semantic meaning. only grammatical, i.e., написать is the pf. of писать. Try Terrence Wade's book.