@Override
  public String getDisplayString(String[] children) {
    return getStandardDisplayString("ndv_compute_bit_vector", children, ",");
  }
GenericUDFNDVComputeBitVector. The ndv_compute_bit_vector function can be used on top of
compute_bit_vector aggregate function to extract an estimate of the ndv from it.
ql/src/java/org/apache/hadoop/hive/ql/udf/generic/GenericUDFNDVComputeBitVector.java