def columns(self):
        """List[:class:`~.external_config.BigtableColumn`]: Lists of columns
        that should be exposed as individual fields.

        See
        https://cloud.google.com/bigquery/docs/reference/rest/v2/jobs#configuration.query.tableDefinitions.(key).bigtableOptions.columnFamilies.columns
        https://cloud.google.com/bigquery/docs/reference/rest/v2/tables#externalDataConfiguration.bigtableOptions.columnFamilies.columns
        """
        prop = self._properties.get("columns", [])
        return [BigtableColumn.from_api_repr(col) for col in prop]