Determining the Optimal Undo Retention Period

You can calculate the actual undo that’s generated in your database by issuing the following query

select sum(d.bytes) "undo"
from v$datafile d,
v$tablespace t,
dba_tablespaces s
where s.contents = 'UNDO'
and s.status = 'ONLINE'
and t.name = s.tablespace_name
and d.ts# = t.ts#;

UNDO
----------------------
122159104000

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You can calculate the value of UNDO_BLOCKS_PER_SEC with the following query:

SQL> select max(undoblks/((end_time-begin_time)*3600*24))
"UNDO_BLOCK_PER_SEC"
 FROM v$undostat;


UNDO_BLOCK_PER_SEC
-------------------------------------------
85.1966666666667

###############################

you can calculate a more precise value for the UNDO_RETENTION parameter.Use the following formula to calculate the value of the UNDO_RETENTION parameter

UNDO_RETENTION = UNDO SIZE/(DB_BLOCK_SIZE*UNDO_BLOCK_PER_SEC)

177515.5545222041 = 122159104000 /(8096*85.19)