general table stats, ideas ?

Hey folks,
I want to do a fairly simple thing, but so far I see no way in which this
could be implemented with postgresql. So I decided to ask folks here.
My problem is, I need to benchmark set of tables, where - we can assume -
schema of each table is unknown, and we have no assumption on any fields
being present there. (altho, if there is no other way to do it, we could
assume id bigint not null default nextval('someseq'));

basically, I need to know when certain row was selected (read), and when it
was updated/created (insert). For that I need two fields.
And although former could be done, and I more or less know how to do it (I
think it can be done with rule, and default = now()) - I have yet to find a
way on how to do the same thing for select.

If someone loves a challenge, and is able to provide me with an answer to my
problem - please do so. Also, if this can be done, but with slightly
different requirements - please let me know too.

Thanks, this is my first post here - so welcome you guys

--
GJ