Undo Space Mangement

Undo space management:

You can look at V$UNDOSTAT view to estimate the amount of undo space required for current workload.
Oracle uses this view to tune undo usage in the system. This view returns NULL if the system is in manual undo managment mode.

Each row in the view keeps statistics collected in the instance for a 10 minute interval. The rows are in descending order; so the first row  has the info about the current time period. The view contains a total of 576 rows max, spanning a 4 day cycle.

It contains following important statistics:

undotsn: the tablespace id of the active undo tablespace
txncount : total number of transactions in 10 minute cycle
maxquerylen: how much time in seconds did the longest query take