Oracle Rollback of Transactions

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Rolling back means undoing any changes to data that have been
performed by SQL statements within an uncommitted transaction. Oracle
uses undo tablespaces (or rollback segments) to store old values. The
redo log contains a record of changes.

Oracle lets you roll back an entire uncommitted transaction.
Alternatively, you can roll back the trailing portion of an
uncommitted transaction to a marker called a savepoint.


All types of rollbacks use the same procedures:
Statement-level rollback (due to statement or deadlock execution
error)
Rollback to a savepoint
Rollback of a transaction due to user request
Rollback of a transaction due to abnormal process termination
Rollback of all outstanding transactions when an instance terminates
abnormally
Rollback of incomplete transactions during recovery


In rolling back an entire transaction, without referencing any
savepoints, the following occurs:
1. Oracle undoes all changes made by all the SQL statements in the
transaction by using the corresponding undo tablespace.
2. Oracle releases all the transaction's locks of data
3. The transaction ends.