The SQL engine part of osquery is backed by SQLite's parser, lexer, query planner, etc. MySQL would be awesome as well though, mostly because of how easy it might be to integrate the MySQL remote access protocol to do remotely authenticated queries!
[1] http://multicorn.org/foreign-data-wrappers/ [2] https://github.com/Kozea/Multicorn/blob/master/python/multic...