title: fetch all partition keys in cassandra
body: <p>I have a high load of data (~ 50 mil per minute) this is how my table looks like:</p>
  
  <pre><code>CREATE TABLE test.metric (
      key text,
      timestamp bigint,
      value double,
      PRIMARY KEY (key, timestamp) )
  </code></pre>
  
  <p>I need to get all unique <code>key</code> . I can do of course <code>SELECT DISTINCT key from metrics;</code> but I was wondering if there is more efficient way (besides creating another table that contains only the keys) to fetch all distinct partitions  keys (maybe from some cassandra table)</p>