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  1. #1
    Join Date
    Aug 2004
    Posts
    26

    Question Unanswered: Undo data storage (Plz, explain if you have time)

    Hi,
    I have a confussion regarding uncommited data if a user not commit an updated row where its before Image and after Image stored, in my knowledge the before image stored in undo tablespace and after image(updated data) will be written on datafile in case of instance faliure or rollback this undo data written back to datafile from undo tablespace, if I am wrong plz correct me.
    Thanks & Regards
    Khawar
    Last edited by skhawar03; 09-19-04 at 11:24. Reason: some guys understand it as REDO data rather then UNDO

  2. #2
    Join Date
    Sep 2002
    Location
    UK
    Posts
    5,171
    Provided Answers: 1
    That sounds broadly correct. You should refer to the Concepts Guide for more details. Also, Tom Kyte's book "Expert One-on-One Oracle" has a whole chapter devoted to this subject.

  3. #3
    Join Date
    Aug 2003
    Location
    Where the Surf Meets the Turf @Del Mar, CA
    Posts
    7,776
    Provided Answers: 1
    >after image(updated data) will be written on datafile
    Sorry, but technically this is not correct.
    The act of a "COMMIT" means the data has been sucessfully written to the REDO logfile. At this point in time there is no guarentee that data has actually be written into the "datafile". This is whole & only purpose for the REDO logfiles.
    The information contained within the REDO logfile allows a transaction to be done again (redo or "roll forward") in the event of a database/system crash.
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