protected BasePartitionEvaluator createPartitionEvaluator(
      WindowFrameDef winFrame,
      PTFPartition partition,
      List<PTFExpressionDef> parameters,
      ObjectInspector outputOI,
      boolean nullsLast) {
    return new BasePartitionEvaluator(this, winFrame, partition, parameters, outputOI, nullsLast);
  }
This class needs to be overridden by the child class to implement function
 specific evaluator.
ql/src/java/org/apache/hadoop/hive/ql/udf/generic/GenericUDAFEvaluator.java