Historically, that was true, but recent versions also allow ad-hoc queries. That being said, VoltDB is no longer the state of the art in serializable OLTP perf

Historically, that was true, but recent versions also allow ad-hoc queries. That being said, VoltDB is no longer the state of the art in serializable OLTP performance (though maybe it is in the commercial space, but I think HyPer for example outperforms it on a lot of benchmarks).