@Override
    public Optional<ColStatistics> estimate(List<ColStatistics> argStats) {
      PessimisticStatCombiner combiner = new PessimisticStatCombiner();
      for (int i = 0; i < argStats.size(); i++) {
        combiner.add(argStats.get(i));
      }
      return combiner.getResult();
    }
GenericUDF Class for SQL construct "COALESCE(a, b, c)".

NOTES: 1. a, b and c should have the same TypeInfo, or an exception will be
thrown.
ql/src/java/org/apache/hadoop/hive/ql/udf/generic/GenericUDFCoalesce.java