Indeed, I'd never heard about it and found myself intrigued. After a bit of reading, it appears that VoltDB supports a very limited number of DDL and DML operations (essentially, you can create tables an views, and select, insert, update, and delete). There are a few basic aggregate functions like SUM and COUNT and no single-row functions. If you need to do anything at all complicated you're doing it in Java, or returning the results to the client program so it can do it.

VoltDB sound like a potentially great thing for an application that's really (and only) OLTP. But it's definitely not ready to replace Oracle, PostgreSQL, etc. for more general-purpose needs.