Undo segment

[dhar_kiran]

undo segments contain pre image or old data and it remains in these segments till another coming entry will not force it to room out, if there is no space in segments. Undo segments provide:

1. Transaction Recovery (at the time of instance recovery )

2. Transaction Rollback (if the transaction is rollbacked, pre image or old data is stored from undo segments.)

3. Read Consistency (when the data is modified in database buffer cache by any DML, its pre image or old data is stored in undo segments, because it is not commited yet, so that users other than the session firing DML could access old data.)

Undo segments didn't contain new data, but both new and old image is stored in redolog files.