def keys(self):
        """D.keys() -> a set-like object providing a view on D's keys"""
        return set(
            row[self._keycol] for row in self._connection.execute(
                """SELECT DISTINCT {} FROM {} ORDER BY {} ASC;""".format(
                    self.selkeycol,
                    self.table,
                    self._keycol
                )
            )
        )
D.keys() -> a set-like object providing a view on D's keys