def get_cursor(self, instance, db_key, db_name=None):
        '''
        Return a cursor to execute query against the db
        Cursor are cached in the self.connections dict
        '''
        conn_key = self._conn_key(instance, db_key, db_name)
        try:
            conn = self.connections[conn_key]['conn']
        except KeyError:
            # We catch KeyError to avoid leaking the auth info used to compose the key
            # FIXME: we should find a better way to compute unique keys to map opened connections other than
            # using auth info in clear text!
            raise SQLConnectionError("Cannot find an opened connection for host: {}".format(instance.get('host')))
        return conn.cursor()
Return a cursor to execute query against the db
        Cursor are cached in the self.connections dict