Possibilities of PgSQL

Hi,

I have a question about PgSQL. I am working at some project and I want
to have few tables with special properties:

There would be many rows and these would be changed every day for many
times. I want to be able to get know how did the row looked in given
time (f.e. 2 day ago, 6 minutes ago, 2nd Sept. 2002,...). Nevertheless
it should work with diffs (f.e. if I had a row with 100MB string and I
changed only 2 chars it should not require on disk 200MB but only 100MB
+ few (kilo)Bytes).

Is there any possibility how to solve it? I have heard that I can do it
with triggers (but the worst thing is how to implement diffs) but I hope
that there is any other (easier) way.

Karel Břinda