Return a hash for the current query.

        This hash is _not_ a unique representation of the dataset!

def _hash(self):
        """Return a hash for the current query.

        This hash is _not_ a unique representation of the dataset!
        """
        dump = dumps(self.query, sort_keys=True)
        if isinstance(dump, str):
            dump = dump.encode('utf-8')
        return md5(dump).hexdigest()