Because there is no answer given yet: You can use pgAdmin to explore an already working database. Maybe you can use this as a start. I have experience with this and it works fine, however it doesn't present you the diagrams. There seems to be also an opensource tool OOOR - Open Object On Rails with which you can produce OpenERP UML diagrams. I have read about it but I have no hands on experience with this OOOR. There is a blog about this: link text There are also Database Master, DBWrench, Rise, SQLPowerArchitect and Navicat. I have also no experience with these tools.

there are plenty of tools for doing the job, google for

PostgreSQL - general - Tool for data modeling and ER diagram

and you will see a very long list of database modeling tools

The main point for us, is that we  want to understand the relations, because we are  working in how we can import our current data, and I think this is necesary to know.

I ask this to all of u that have experience with this great software.

What tools do u recommend us to start learning how each table is connected, I think is important right?