@Override
  public StatEstimator getStatEstimator() {
    return new WhenStatEstimator();
  }
GenericUDF Class for SQL construct
"CASE WHEN a THEN b WHEN c THEN d [ELSE f] END".

NOTES: 1. a and c should be boolean, or an exception will be thrown. 2. b, d
and f should be common types, or an exception will be thrown.
ql/src/java/org/apache/hadoop/hive/ql/udf/generic/GenericUDFWhen.java