Open Source solutions are probably the only place where you'll find this information fully disclosed. Proprietary software vendors usually don't reveal much about the database because A) they reserve the right to change the internals at any time B) their preferred / officially supported method for customers to access the data is through the application interfaces provided and not directly from the database.
There is a third party solution that exposes some of the metadata from the SAP database model. I've not used this myself however. CA ERwin Saphir Option Demo