public static function fragments($expiration=7200) {
		$sql = self::$sql;
		$args = [];
		$args['index'] = $sql->stmt_fragment('index');
		$args['engine'] = $sql->stmt_fragment('engine');
		$args['dtnow'] = $sql->stmt_fragment('datetime');
		$args['expire'] = $sql->stmt_fragment(
				'datetime', ['delta' => $expiration]);
		if ($sql->get_connection_params()['dbtype'] == 'mysql')
			$args['dtnow'] = $args['expire'] = 'CURRENT_TIMESTAMP';
		return $args;
	}