How to tackle a large undocumented database I recently got hired as the only IT Guy in a certain Company X and I am tasked to fix their applications, and in my opinion, the best way to start is by understanding the database.

Their current database is a MySQL database with 186 tables(note that some tables are empty for god knows why). And the application is communicating with the database through an MS Access database interface. (I ask myself why the developers did that too)

Question is, How do I start tackling this large undocumented database? Yes, it is undocumented because the application's developers aren't willing to give me an ERD or data dictionary or any information at all about the database to make my life easy. How would you suggest to take on this perilous endeavor of understanding every nook and cranny of the rather large database?

Related Question: How to dive into an ugly database?
The linked answer tackles the problem bottom-up, database first. As your responsibilities encompass the applications and database, I'd be inclined to attack this top-down starting at the application(s).

Focus your attention on understanding the most frequently used features of the application through consultation with the user base. Trace the database interactions of those features through profiling/logging tools so you can identify the key tables and procedures. 

This way your early efforts are constrained to the "stuff that matters", rather than wasting time documenting tables and queries that may be rarely or never used. The focus should also bring the Pareto Principle to bare on your bug fixing efforts (so says Microsoft anyway).