Redo and Undo Generation 

To find sessions generating lots of redo, you can use either of the following 

methods. Both methods examine the amount of undo generated. When a transaction 

generates undo, it will automatically generate redo as well.

The methods are:

1) Query V$SESS_IO. This view contains the column BLOCK_CHANGES which indicates

   how much blocks have been changed by the session. High values indicate a 

   session generating lots of redo.

   The query you can use is:

       SQL> SELECT s.sid, s.serial#, s.username, s.program,

         2  i.block_changes

         3  FROM v$session s, v$sess_io i

         4  WHERE s.sid = i.sid

         5  ORDER BY 5 desc, 1, 2, 3, 4;

Run the query multiple times and examine the delta between each occurrence

   of BLOCK_CHANGES. Large deltas indicate high redo generation by the session.

2) Query V$TRANSACTION. This view contains information about the amount of

   undo blocks and undo records accessed by the transaction (as found in the 

   USED_UBLK and USED_UREC columns).

  The query you can use is:

      SQL> SELECT s.sid, s.serial#, s.username, s.program, 

        2  t.used_ublk, t.used_urec

        3  FROM v$session s, v$transaction t

        4  WHERE s.taddr = t.addr

        5  ORDER BY 5 desc, 6 desc, 1, 2, 3, 4;

   Run the query multiple times and examine the delta between each occurrence

   of USED_UBLK and USED_UREC. Large deltas indicate high redo generation by 

   the session.

You use the first query when you need to check for programs generating lots of 

redo when these programs activate more than one transaction. The latter query 

can be used to find out which particular transactions are generating redo.