Rollback Segment vs Redo log
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Thread: Rollback Segment vs Redo log

  1. #1
    Join Date
    Jun 2006
    Posts
    101

    Rollback Segment vs Redo log

    Hi guys, I have come across below paragraph today

    A Rollback Segment is a database object containing before-images of data written to the database. Rollback segments are used to:
    Undo changes when a transaction is rolled back
    Ensure other transactions do not see uncommitted changes made to the database
    Recover the database to a consistent state in case of failures
    My understanding is that Oracle also stores before image for committed and uncommitted transactions in the redo-log files! So the question is why does Oracle need rollback segment? Why cant Oracle provide rollback functionality/read consistency and recovery by using the redo log files?
    Thanks!

  2. #2
    Join Date
    Nov 2002
    Location
    Mooresville, NC
    Posts
    349
    For back up recovery Oracle uses Redo logs.

    Before data committed you need Undo segments data.
    After commit you need Redo log data to recover.
    http://www.perf-engg.com
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  3. #3
    Join Date
    Sep 2002
    Location
    England
    Posts
    7,331
    Quote Originally Posted by khoshtip_uk View Post
    Hi guys, I have come across below paragraph today

    A Rollback Segment is a database object containing before-images of data written to the database. Rollback segments are used to:
    Undo changes when a transaction is rolled back
    Ensure other transactions do not see uncommitted changes made to the database
    Recover the database to a consistent state in case of failures
    My understanding is that Oracle also stores before image for committed and uncommitted transactions in the redo-log files! So the question is why does Oracle need rollback segment? Why cant Oracle provide rollback functionality/read consistency and recovery by using the redo log files?
    Thanks!
    that is one of the most basic concepts of oracle - in fact the most important one

    go and read the concepts manual

    your statement about redo logs is very very wrong

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