@Override
    public AggregationBuffer getNewAggregationBuffer() throws HiveException {
      PercentileAggBuf result = new PercentileAggBuf();
      result.histogram = new NumericHistogram();
      reset(result);
      return result;
    }
Construct a histogram using the algorithm described by Ben-Haim and Tom-Tov, and then
use it to compute an approximate percentile value.
ql/src/java/org/apache/hadoop/hive/ql/udf/generic/GenericUDAFPercentileApprox.java