Unpack rows for data (from a select statement) from payload and yield a single row at a time.
        :param column_types: a tuple of column descriptors
               e.g. (<class 'pyhdb.protocol.types.String'>, <class 'pyhdb.protocol.types.ClobType'>)
        :param connection: a db connection object
        :returns: a generator object

def unpack_rows(self, column_types, connection):
        """Unpack rows for data (from a select statement) from payload and yield a single row at a time.
        :param column_types: a tuple of column descriptors
               e.g. (<class 'pyhdb.protocol.types.String'>, <class 'pyhdb.protocol.types.ClobType'>)
        :param connection: a db connection object
        :returns: a generator object
        """
        for _ in iter_range(self.num_rows):
            yield tuple(typ.from_resultset(self.payload, connection) for typ in column_types)