Information of V$instance
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Thread: Information of V$instance

  1. #1
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    Where is the information of V$INSTANCE view comes from ?? IS it comes from the controlfiles??
    If it comes from the control files then does controlfiles stores the value of machine name etc.??

  2. #2
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    http://otn.oracle.com/docs/products/...ch368.htm#8121

    v$instance is a synonym that points to a view ( v$_instance ). You can get the source of the view by looking a dba_views.

    The host_name is not stored in the control file.
    Jeff Hunter
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  3. #3
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    My actual question was weather V_$INSTANCE Fetch the data from control files??
    IF not then from where does it actually fetches the data ???

  4. #4
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    v$ is based on x$
    some infos v$ gathers comes from control file
    some infos v$ gathers comes from memory structure

    does this answer?

  5. #5
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    Originally posted by marist89

    The host_name is not stored in the control file.
    Jeff Hunter
    marist89@yahoo.com
    http://marist89.blogspot.com/
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  6. #6
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    Pando,
    You got my question , can you tell me weather v$instance info is coming from memory structures or from control file?
    An elephant is a mouse with an operating system.

  7. #7
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    well I cant be sure on this one but i would say it comes both memory structure and control file
    the problem with x$ views is that there is virtually 0 documents about them therefore the only way to understand some of them is investigate yourself by dumping memory structure for example

  8. #8
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    It is for sure that information from V$INSTANCE comes solely from contolfile -including with the hostname (despite the fact that Jeff repeated the oposite twice! ).

    Any information that any V$ view shows comes from three sources:

    a) from control file
    b) from data dictionary stored in a system tablespace
    c) from the statistics gathered by the database instance while the database is opened

    So which V$ views get their information entirely from controlfile? This is very easy to define:

    - Start your database in MOUNT mode
    - Query from that particular V$ view

    If you get any meaningful output you can be sure tha information comes entirely from the controlfile. When database is in MOUNT stage, only information from controlfile is available, while data dictionary (residing in SYSTEM tablespace) is not yet opened - so information could come only from controlfile.

    And such is a case with V$INSTANCE (including the hostname it reports).

    [Edited by jmodic on 01-11-2002 at 06:04 PM]
    Jurij Modic
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  9. #9
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    I thought memory stats must come from memory structure? Such as v$latch, v$session_wait etc

  10. #10
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    Ups, I think I must bite myself in my own tongue! There is another source of information for V$ views - operating system! And host name is realy not stored in a controlfile (if this was not the case, controlfiles would not be so easily transportable among different machines with the same operating system), V$INSTANCE view gets that from an operating siystem! Othere V$INSTANCE values are mainly gotten from the controlfile though (instance_number, instance_name, version, startup_time, status, etc...).

    So Jeff was right after all - hot name is not stored in the control file.
    Jurij Modic
    ASCII a stupid question, get a stupid ANSI
    24 hours in a day .... 24 beer in a case .... coincidence?

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