private FunctionInfo registerUDAF(String functionName, FunctionType functionType,
      Class<? extends UDAF> udafClass, FunctionResource... resources) {
    validateClass(udafClass, UDAF.class);
    FunctionInfo function = new WindowFunctionInfo(functionType, functionName,
        new GenericUDAFBridge(ReflectionUtil.newInstance(udafClass, null)), resources);
    addFunction(functionName, function);
    addFunction(WINDOW_FUNC_PREFIX + functionName, function);
    return function;
  }
Registers the UDF class as a built-in function; used for dynamically created UDFs, like
GenericUDFOP*Minus/Plus.
ql/src/java/org/apache/hadoop/hive/ql/exec/Registry.java