@Override
  public GenericUDAFEvaluator getEvaluator(TypeInfo[] parameters)
      throws SemanticException {
    // This method implementation is preserved for backward compatibility.
    return new GenericUDAFCountEvaluator();
  }
This class implements the COUNT aggregation function as in SQL.
ql/src/java/org/apache/hadoop/hive/ql/udf/generic/GenericUDAFCount.java