def _read_xml_db(self):
        """
        read metadata from an xml string stored in a DB.

        :return: the root element of the xml
        :rtype: ElementTree.Element
        """
        try:
            metadata_str = self.db_io.read_metadata_from_uri(
                self.layer_uri, 'xml')
            root = ElementTree.fromstring(metadata_str)
            return root
        except HashNotFoundError:
            return None
read metadata from an xml string stored in a DB.

        :return: the root element of the xml
        :rtype: ElementTree.Element