Log Sequence Number vs System Change Number
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Thread: Log Sequence Number vs System Change Number

  1. #1
    Join Date
    Mar 2007
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    21

    Log Sequence Number vs System Change Number

    Hi All,

    I don't know what's difference between Log Sequence Number and System Change Number?
    When's Log Sequence Number updated or increased?
    When's System Change Number updated or increased?

    Thanks in advance,
    Sinh

  2. #2
    Join Date
    Aug 2007
    Location
    Cyberjaya,kuala lumpur
    Posts
    339
    log sequence number
    A number that uniquely identifies a set of redo records in a redo log file. When Oracle fills one online redo log file and switches to a different one, Oracle automatically assigns the new file a log sequence number. For example, if you create a database with two online log files, then the first file is assigned log sequence number 1. When the first file fills and Oracle switches to the second file, it assigns log sequence number 2; when it switches back to the first file, it assigns log sequence number 3, and so forth.

    System Change Number - A number, internal to Oracle that is incremented over time as change vectors are generated, applied, and written to the Redo log.

  3. #3
    Join Date
    Mar 2007
    Posts
    21
    Thanks for your replying. It's clearly about sequence change number.

    But I have 1 more question : When's SCN changed? When commit, checkpoint happen or Add tablespace then SCN is changed or not?

    Thanks in advance,
    Sinh

  4. #4
    Join Date
    Aug 2007
    Location
    Cyberjaya,kuala lumpur
    Posts
    339
    The SCN is incremented whenever a transaction commits. However, this is not the only source of increments. In a seemingly idle database, the SCN gets incremented also through AQ, SMON, job queues...

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