Ubuntu 12.04 is the current LTS version of the distribution and is what I'm using on most of the servers I get to manage.
I always try to work with the packages provided by the distribution. In this case Ubuntu bundled PostgresSQL 9.1 as the most up-to-date version of the database software in the distribution (you can always use an update PPA to get the shiny new things).
In the middle of a small migration between versions of our software I needed to populate a database with information from other database. By design you can't do queries between databases in PostgreSQL because it loads database-specific system catalogs and it is uncertain how a cross-database query should even behave.
For this problem you can use dblink, it basically acts like a function that contains a query in it and returns the result of the query. dblink comes as part of the contrib package in PostgreSQL, and it's present in the postgresql-9.1-contrib package in Ubuntu.
So, supposing you already have PostgreSQL 9.1 running you can install the contrib package with:
sudo apt-get install postgresql-contrib-9.1
And to load the dblink modules in PostgreSQL you've to:
sudo su postgres -c "psql -c "CREATE EXTENSION dblink" postgres"
Supposing that "postgres" is the database where you want to use dblink. That's it, to try it:
postgres=# select username, app from dblink('dbname=booki', 'select usename, application_name from pg_stat_activity') as t(username text, app text); username | app ----------+------ postgres | psql postgres | (2 rows)