MongoDB (and SQL dbs) does ad-hoc queries quite well as long as your data fits on a single machine. Cassandra does ad-hoc queries quite well if you're willing

MongoDB (and SQL dbs) does ad-hoc queries quite well as long as your data fits on a single machine. Cassandra does ad-hoc queries quite well if you're willing to use mapreduce. If you have a smaller number of frequently done queries and are willing to use MR for the others maybe that is a happy place for you.