Find the amount of redo generated by the block changes column high value

It will bring the top most line because of Order by clause on i.Block_changes in descending order.

SELECT s.sid, s.serial#, s.username, s.program, i.block_changes
FROM v$session s, v$sess_io i
WHERE s.sid = i.sid
ORDER BY 5 desc, 1, 2, 3, 4;

Find the amount of undo blocks and undo records accessed by the transaction

SELECT s.sid, s.serial#, s.username, s.program,t.used_ublk, t.used_urec
FROM v$session s, v$transaction t
WHERE s.taddr = t.addr
ORDER BY 5 desc, 6 desc, 1, 2, 3, 4;