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  1. #1
    Join Date
    Jan 2010

    Unanswered: Read Consistency provide by SGA or Undo data file

    I am confused as to where the old image of data is stored once it has been modified? I know that the change vectors go into the redo log buffer and the modified buffer is in the database buffer cache and I have read in several places that the old image ins kept in undo block...but I am confused as to where does this undo block reside.. Is it in Database Buffer Cache or somewhere else? If its in the database buffer cache then do they ever get written to the Undo datafile? Supposing they do get written then I fail to understand the advantage of using UNDO tbs for read consistency as reading the data block from the undo data file and reading the block from the data file will both involve disk I/O (I am taking into account that the modified data hasn't be written into the data file yet). Kindly help me out.

  2. #2
    Join Date
    Aug 2003
    Where the Surf Meets the Turf @Del Mar, CA
    Provided Answers: 1
    You can lead some folks to knowledge, but you can not make them think.
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