code: def do_execute(self):
          
          iquery = InstanceQuery()
          iquery.db_url = str(self.resolve_option("db_url"))
          iquery.user = str(self.resolve_option("user"))
          iquery.password = str(self.resolve_option("password"))
          props = str(self.resolve_option("custom_props"))
          if (len(props) > 0) and os.path.isfile(props):
              iquery.custom_properties = props
          iquery.query = str(self.resolve_option("query"))
          data = iquery.retrieve_instances()
          self._output.append(Token(data))
          return None
docs: The actual execution of the actor.
  
          :return: None if successful, otherwise error message
          :rtype: str