I think I've heard of this akonadi. That's the thing that isn't actually a
DB, but backends to one, right? Iirc, it's the cause of some ire in Debian
because the package depends on the mysql-server package, meaning any KDE
desktop user needs an installed and configured mysql server on their
Anyway, I think xapian is more fit-for-purpose for this problem than a
generic relational database.