of course I've heard of column store relational databases, but they're not nearly as efficient nor scalable as Cassandra which has been built from the ground up for this. Fact is, if you don't need referential integrity, there is absolutely no need to pay the massive complexity overhead and horizontal scalability penalties that the relational databases incur.

Separately it is absolutely correct that a comment that asserts that 4 relational databases are the only serious ones is completely misguided and deserves a robust rebuke.