code: private ByteBuffer getRawKeyFromRowGroup(Iterable<Row> rowGroup) {
          Iterator<Row> iter = rowGroup.iterator();
          // Sanity check
          assert iter.hasNext() : "Row group should never contain zero rows";
          return getKey(iter.next());
      }
docs: A few notes on this method:
  <ol>
  <li>All rows in the row group have the same key, so choosing the first row is safe.</li>
  <li>The rowGroup will always contain at least one row.</li>
  <li>The row group has at least the first row in hard cache, so iterating to the first row will never
  result in a new CQL query.</li>
  </ol>