> > I recommend to install the latest major version and the major version
> > behind that to make the upgrading of the database easier.
> 
> I assume you mean two versions of PostgreSQL?
> 
> Can you elaborate a little on why / how this helps with upgrading the
> database?

Yes the current major version  of your distro and the older one.

A little explenation why you need to upgrade databases in postgres: because 
the format is not stable between major versions you have create a new db for 
the new major version and import the old one.

Why it is useful to also  have the older one:
If you have the older one also installed you can use pg_upgrade to migrate a 
postgresql database automaticly.
Akonadi uses that when installed.
If not you have to dump the older db first before updating postgresql and then 
import the dump into the database