public function get_sort_columns() {
        $result = parent::get_sort_columns();

        $assignment = $this->assignment->get_instance();
        if (empty($assignment->blindmarking)) {
            $result = array_merge($result, array('userid' => SORT_ASC));
        } else {
            $result = array_merge($result, [
                    'COALESCE(s.timecreated, '  . time()        . ')'   => SORT_ASC,
                    'COALESCE(s.id, '           . PHP_INT_MAX   . ')'   => SORT_ASC,
                    'um.id'                                             => SORT_ASC,
                ]);
        }
        return $result;
    }