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What does "before image" mean?

Discussion in 'SQL PL/SQL' started by preston, May 14, 2012.

  1. preston

    preston Active Member

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    I am just reading an article relevant to oracle recovery, "after image" or "before image" words have been used. I don't understand what does it mean do u have any ides?

    Thanks
     
  2. zargon

    zargon Community Moderator Forum Guru

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    That depends on the article; Oracle recovery in what sense? From a valid backup or after a crash? Normally refering to before image and after image implies the data state before and after a change, such as an update. The redo logs record both 'images', as change vectors, so that the executed change can either be committed and restored if a crash or recovery is required or rolled back should the inssert/update/delete be in error. Thus the data is committed to the state after the change (after state) or rolled back to its original state before the chanje (before state).

    Not having the article in front of me that's the best explanation I can provide.
     
  3. preston

    preston Active Member

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    The subject of article is recovery for crash state. So that means "change", right?
     
  4. zargon

    zargon Community Moderator Forum Guru

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    Yes, and my explanation is what you needed to know.
     
  5. preston

    preston Active Member

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    Thanks a lot mate.
     
  6. aclink

    aclink Active Member

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    thank you for explaning. i know a lot now