I've also downloaded the source code to many dbms but they're all far too complex for me to get a good idea of what's going on just by reading the source.

Even DBD::SQLite? I think you really should consider studying existing database systems. If they seem overly complex that's probably because building a database, even a pretty modest one, is a lot of work. If you ignore the many fine examples of the art you'll just have to discover them the hard way!

-sam