Re: Bookish Questions need Ideas

"Arijit Chatterjee" <arijitchatterjee123_at_yahoo.co.in> wrote in message news:ea01504d.0409300314.43ea03f7_at_posting.google.com...
| Dear Faculties..
| Again some basic question I am asking regarding
| Rolllback Segment and Redo Log.Yes this time after
| reading from many resources.But last time also I read that
| but why multiple files are using that was the main
| question.
|
| Point1 : A rollback segment records the old values of data
| that was changed by each transaction (whether or not committed).
|
| Point 2 : The redo log records all changes made to data,
| including both uncommitted and committed changes.
|
| Q1.So any Transaction Rollback there are data duplication storage
| is happening.Means Rollback segment is storing the data as well
| as the rego log.Is it true?If true then why?
|
| Q2.But I have a doubt on my question also.Is rollback segment stores
| data or refresh it after stipulated hours or after every load???
|
| Regards
| Arijit Chatterjee

rollback segments are created for rollback (undo) operations

redo logs are created for redo operations

if a transaction needs to be rolled back, the data stored in rollback segments is used to restore database data to its prior state (basically oracle makes changes directly to the 'live' data, but stores the old data in rollback segments in case the changes are not committed)

if the (part of) the database needs recovery, all transaction activity is stored in the redo logs -- which can be archived -- so that old copies of database files can be used as a starting point to rebuild (re-apply, or re-do) all transactions during database or datafile recovery

additionally, the rollback segments provide multi-version read consistency -- if user A makes changes, SQL statements issued by user B still see the old data (retrieved from the rollback segments) until after user A commits his/her transaction

keep reading!

++ mcs Received on Thu Sep 30 2004 - 06:50:37 CDT