Finding daily REDO

Script To Find Redolog Switch History And Find Archivelog Size For Each Instances In RAC (Doc ID 2373477.1).

Looking at the above document, it says to use FIRST_TIME only. The output has major difference also while we use FIRST_TIME compared to COMPLETION_TIME.

Why there is a difference please?

SQL> select count  (*) "Redo Count",

   trunc(completion_time) "Date",

   round(sum(blocks*block_size)/1024/1024/1024,2) "Amount of Redo in Gb"

   from ( select distinct name, blocks, block_size, completion_time from v$archived_log )

   where trunc(completion_time) > sysdate - 20

   group by trunc(completion_time)

   order by 2 desc;

Redo Count Date      Amount of Redo in Gb