title: How to choose Cassandra version?
body: <p>I'm sql coder and new in Cassandra, which version should I choose, Cassandra 3.X or Cassandra 2.X? </p>
  
  <p>We already have cassandra-2.1.17 but it could not support any aggregate function at all, I want to upgrade to cassandra 3.X but I don't know much about the difference, just want to get some advice, appreciate.</p>