def execute_sql(self, statement):
        """
        Executes a single SQL statement.

        :param statement: SQL string
        :return: String response
        :rtype: str
        """
        path = '/archive/{}/sql'.format(self._instance)
        req = archive_pb2.ExecuteSqlRequest()
        req.statement = statement

        response = self._client.post_proto(path=path,
                                           data=req.SerializeToString())
        message = archive_pb2.ExecuteSqlResponse()
        message.ParseFromString(response.content)
        if message.HasField('result'):
            return message.result
        return None