The possible causes for Excessive Undo generation

1)Transactions with huge undo.
-------------------------------
It is obvious to see high undo usage when there are huge transactions. If that is going to be the case this growth should be expected behavior.

2)The Higher value setting of UNDO_RETENTION.
------------------------------------------------
The higher value of UNDO_RETENTION will increase much undo. As it can't be marked undo extents as EXPIRED till the duration of the UNDO_RETENTION.

3)Autoextensible mode of Undo data files.
--------------------------------------------------
Disabling the auto extensible mode of the datafiles of active undo tablespace will reuse the UNEXPIRED extents when undo tablespace suffer in space pressure.If they are set to auto extensible, it will not reuse the space for the new transaction. In such scenarios new transaction will allocate a space and your undo tablespace will start growing.

4)Undo tablespace is using Autoallocate option:
----------------------------------------------
UNDO is using auto allocate option of LMT. As the number of the extents goes up, the extent size will be increased too.When the number is extents reaches hundred the extent size will be bigger. If it reaches thousands it will be even more bigger.If you know the size of most of your transactions you can use UNIFORM rather than AUTO.


How Undo Allocation Done