Undo segment

[dilipbabu_184]

Hi,
I have a question and i really confused with it

What does Undo segments contain?. I know it contains the old values till the transaction is committed. Does it contain the newly updated values along with the old values till it is commited? and what else it contains..

regards,
Dilip

[davey23uk]

it contains the old values so that a read consistent view can be provided

changes to the data are store in the redo logs

[tabreaz]

Undo Segments contains old values which will allow to rollback transaction, provide read consistancy and to recover the database.

[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.