Monitoring database for changes - backup purposes

Hi,

I have a query about a backup policy that I wish to implement.
Currently, we are running a PostgreSQL server with a very large database
on it.  The database consists of a core of read-only tables, with tables
created by the users going by default into the user's own schemas.  For
backup purposes, I have a script that uses pg_dump to create dumps of
the individual user schemas.  However, this is mainly a read-only
database and information is added or updated only rarely.  What I really
need is a method of monitoring the database, such that my backup script
will only back any one schema up if it has changed since it was last
backed up.  Is there any way to do this?  Is there a checksum function
or something that I could use?

Thanks,

Chris

--
Chris Jewell, BSc(Hons), BVSc, MRCVS
Dept of Maths and Statistics
Fylde College
Lancaster University
Lancaster
Lancs
LA1 4YF