Monitoring activities of PostgreSQL

I do have a PL/SQL function that gets executed called many times but
with different parameter values each of these times. For most
invocations of this function run in a couple of seconds however some
invocations of the same function run (on the same dataset) for hours
with very little disk activity but high CPU.

How can I monitor the actual DB activities during such times so I may
better understand what the situation truly is. I have seen some users
on this list posting some complex log/outputs, this are the kind of
outputs I would like to capture and view. Where are they?

Allan.