What are the differences between SingleStore DB and Spark SQL?

Spark SQL treats datasets (RDDs) as immutable - there is currently no concept of an INSERT, UPDATE, or DELETE. You could express these concepts as a transformation, but this operation returns a new RDD rather than updating the dataset in place. In contrast, SingleStore DB is an operational database with full transactional semantics.

SingleStore DB supports updatable relational database indexes. The closest analogue in Spark is IndexRDD, which is currently under development, and provides updatable key/value indexes.