set @new_etl_start_time_stamp = (SELECT Time_stamp FROM `roll_up_day_physical_index_202106_bak` ORDER BY Time_stamp DESC LIMIT 1);

这是一条MySQL语句，它的作用是从`roll_up_day_physical_index_202106_bak`表中获取最新一条记录的时间戳并将其赋值给变量`@new_etl_start_time_stamp`。具体来说，它通过`ORDER BY Time_stamp DESC`将表中的记录按时间戳从大到小排序，然后通过`LIMIT 1`限制只返回一条记录，最后通过`SELECT Time_stamp`选取时间戳这一列的值。