Database schema details for custom report building?

I'm the IT guy for my wife's store. They're starting to get impatient with the limitations of the built-in and "Extra" reports, and I'm interested in building some new ones for them. I'd much rather spend the time and effort to create custom reports than spend it converting them to a different POS.

That said, it appears that the only supported options for developing custom reports are to either open a request to the Checkout team or to tweak existing reports via trial and error. Both of those approaches seem really inefficient, so I'm hoping you can help me out.

It would save me a TON of time if I could test out come basic SQL queries before building out the full report code and views. Can you share the database schema details or a technique for connecting to a (backup) Checkout database from a Postgres client? Note that the latter would be most helpful, since then I'd be able to look at the schema details myself. I'm familiar with MySQL, but haven't used Postgres before; and my initial attempts to access a Checkout DB via the command-line 'psql' client have failed.