@Override
    public AggregationBuffer getNewAggregationBuffer() throws HiveException {
      return new HypotheticalSetDenseRankBuffer();
    }
Evaluator for calculating the dense rank.
SELECT dense_rank(expression1[, expressionn]*) WITHIN GROUP (ORDER BY col1[, coln]*)
Implementation is based on hypothetical rank calculation but the group of values are considered distinct.
Since the source of the input stream is not sorted a HashSet is used for filter out duplicate values
which can lead to OOM in large data sets.
ql/src/java/org/apache/hadoop/hive/ql/udf/generic/GenericUDAFDenseRank.java