private FunctionInfo registerGenericUDAF(String functionName, FunctionType functionType,
      GenericUDAFResolver genericUDAFResolver, FunctionResource... resources) {
    validateDescription(genericUDAFResolver.getClass());
    FunctionInfo function =
        new WindowFunctionInfo(functionType, functionName, genericUDAFResolver, 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