Noel Faux <noel.faux@med.monash.edu.au> writes:
> We generally only vacuum
> tables which are affected by deletes, inserts and updates.
You can't do that as an exclusive practice :-(. In particular I suppose
that you never vacuumed your system catalogs at all, and now you are
behind the eight ball because transaction IDs wrapped around. See
http://www.postgresql.org/docs/7.4/static/maintenance.html#VACUUM-FOR-WRAPAROUND
regards, tom lane