Cassandra and many other NoSQLs are designed primarily for OLTP workloads, not OLAP. OLTP is almost exclusively "find me sth by primary key" (see the TPC-C benchmark used for RDBMSes). Sorting huge amounts of data, top-n queries, skyline queries, aggregation, joining huge data sets, complex filtering belong to analytics world, not OLTP. Unless your whole database is very tiny, let's say 10MB, those operations are pretty expensive even in RDBMSes. That's why those features are deliberately not included in Cassandra. Contrary, MongoDB took a different route - they include some of those features and then seriously underdeliver on many of them.