http://robsanheim.com/2006/03/10/searching-for-the-perfect-in-memory-database/ mentions derby, hsql, sqllite
I think this kind of thing might be relevant to our systems:
status-quo: We have sybase/udb. I think we use temp tables or staging tables to hold temporary data. a little slow.
status-quo: We create custom objects (java collections) to hold stuff in memory, but need to build custom-methods to access them. A pain to test. Great if we can use standard sql instead of those methods. The idea is to leverage on existing standard and avoid custom coding to duplicate existing standard.
We have lots of feeds.