Closed-source licenses are provided per request and the only restriction seems to be that you don't sell a DB system backed by the lib. If you really want to prevent closed-source abuse of your library, a GPL-based dual licensing setup is the only way I can think of.
Ah -- ok -- I got my info from the COPYING file in the git repo.
Regardless: "those which are distributed and marketed as database systems to be used by other developers" looks to be _full_ of wiggle room within what I suspect is the authors intent.
 "I/somebody will/might make this a higher-order database tool for developers and I don't want anyone to compete with anybody else without forcing everybody involved (with this code) to open their entire codebase" (??)