[matthewmacleod]

You almost certainly want Postgres, unless you have a compelling, specific use case to use a database optimised for a specific workload, or for some reason Postgres isn't usable in your specific environment. It's a reliable, well-designed general-purpose RDBMS which will scale up pretty well to cope with fairly large workloads; performance and latency will not be problems with a sensible schema.

VoltDB, as an example, is very different: it seems to be designed for simple OLTP workloads. It's an in-memory database, which offers opportunities for impressive performance, but if you have a large amount of data, you'll need a large amount of memory. And horizontal scaling is cool, but cross-partition operations will incur significant overhead

(IANAL, but the AGPL requires network users of software be able to download the source. Since a presumably proprietary application is the client in this case, this isn't likely to be an issue.)