@Override
  public StatEstimator getStatEstimator() {
    return new CoalesceStatEstimator();
  }
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