Undo Tablespace/Undo Management in Oracle:

Oracle Database keeps records of actions of transactions, before they are committed and Oracle needs this information to rollback or undo the changes to the database. These records are called rollback or undo records.

These records are used to:

Rollback transactions - when a ROLLBACK statement is issued, undo records are used to undo changes that were made to the database by the uncommitted transaction.

Recover the database - during database recovery, undo records are used to undo any uncommitted changes applied from the redolog to the datafiles.

Provide read consistency - undo records provide read consistency by maintaining the before image of the data for users who are accessing the data at the same time that another user is changing it.

UNDO_RETENTION:

This value specifies the amount of time, undo is kept in the tablespace.The parameter undo_retention to set the amount of time you want undo information retained in the database.

The default value for the UNDO_RETENTION parameter is 900.

If an active transaction requires undo space and the undo tablespace does not have available space, then the system starts reusing unexpired undo space (if retention is not guaranteed). This action can potentially cause some queries to fail with the ORA-01555 "snapshot too old" error message.

UNDO_MANAGEMENT = AUTO