Nobody has mentioned Postgres 10 which is going to be pretty cool in its release - https://www.ashnik.com/new-features-coming-in-postgresql-10/

Postgres is creeping up on Cassandra faster than Cassandra becomes usable. Especially the really cool work that Citus is doing.

Nah, postgresql still doesn't have a nice/easy sharding policy. Don't forget that citus is still single-master (the opensource one). Postgresql-xl also doesn't have nice/easy replication.

Scylladb is creeping up on Cassandra though.

https://github.com/citusdata/citus

Which is why Postgres is creeping up on both scylladb and cassandra.

Plus PG 10 will have declarative partitioning built in. Pretty cool.