def _query_attr_sort_fn(self, attr_val):
        """Used to order where keys by most selective key first"""
        attr, v = attr_val
        if attr in self._indexes:
            idx = self._indexes[attr]
            if v in idx:
                return len(idx[v])
            else:
                return 0
        else:
            return 1e9
Used to order where keys by most selective key first