interview question perhaphs :-)
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  1. #1
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    interview question perhaphs :-)

    Hi

    I am a OCP DBA :-) and i do not know the answers to this questions..all of you are welcome to cricize me :-) for askin this simple question..but i have serached the docs before askin this ?

    here goes my 2 questions

    1)when is the SCN assigned to a transaction..immediately after a commit is issued or after the contents of the redolog buffer are flushed to the disk

    2)when you have row migration why does oracle keep a pointer in the block to the row where it has moved now ?

    regards
    Hrishy

  2. #2
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    Re: interview question perhaphs :-)

    Originally posted by hrishy

    1)when is the SCN assigned to a transaction..immediately after a commit is issued or after the contents of the redolog buffer are flushed to the disk
    Whenever a transaction is committed, LGWR writes the transactionís redo records from the redo log buffer of the SGA to an online redo log file, and a system change number (SCN) is assigned to identify the redo records for each committed transaction.

    Originally posted by hrishy

    2)when you have row migration why does oracle keep a pointer in the block to the row where it has moved now ?
    Only reason i think is to preserve the rowid. The rowid never changes for a row

    HTH
    Amar
    "There is a difference between knowing the path and walking the path."

    Amar's Blog  Get Firefox!

  3. #3
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    Re: Re: interview question perhaphs :-)

    Originally posted by adewri


    Only reason i think is to preserve the rowid. The rowid never changes for a row

    HTH
    Hi Amar.

    Dude thats a cool answer...even i couldnt think of any other answer..

    regards
    Hrishy

  4. #4
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    Re: interview question perhaphs :-)

    Originally posted by hrishy
    Hi

    2)when you have row migration why does oracle keep a pointer in the block to the row where it has moved now ?

    regards
    Hrishy
    It's especially essential if the row is in a table that is a Master table for a materilized view (using ROWID), otherwise, every time you had a row migration, you'd have to rebuild the materialized view.

    Just one example...

    Don't worry about asking questions, even DBA's with 10+ years experince don't know everything. If they did.. it'd be a pretty boring job.
    OCP 8i, 9i DBA
    Brisbane Australia

  5. #5
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    Hi Jhonson

    Thats cool..now next time probably i would get a job..:-)..

    regards
    Hrishy

  6. #6
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    Re: Re: interview question perhaphs :-)

    Originally posted by adewri
    Only reason i think is to preserve the rowid. The rowid never changes for a row
    Presumably if the rowid did change, all the indexes pointing to it would have to be updated too.
    "The power of instruction is seldom of much efficacy except in those happy dispositions where it is almost superfluous" - Gibbon, quoted by R.P.Feynman

  7. #7
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    Exactly as greg said,
    You can trap the best of the dba's with some nice tricky question.

    and trust me answering questions on the net is easy, ask me anything, i'd hunt the net and articulate my ass out of it.

    I think dba/developer interviews should be more of having the candidate sit behind a simple box and gradually answer the questions on the machine.
    That way you know if you want a junior, intermediate or a senior dba/developer.

    That way both the parties know what they've got.
    Tarry Singh
    I'm a JOLE(JavaOracleLinuxEnthusiast)
    TarryBlogging
    --- Everything was meant to be---

  8. #8
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    Originally posted by Tarry
    Exactly as greg said,
    You can trap the best of the dba's with some nice tricky question.

    and trust me answering questions on the net is easy, ask me anything, i'd hunt the net and articulate my ass out of it.

    Another sign of a good DBA: Might not know the answer, but knows where to find it. Very important...

    I sometimes (ahem, often) see questions posted on this board that I don't know the answer to, but I become curious and wonder what the answer is myself, so I usually go searching for it via Metalink, or books. Once I find the answer I post it here. So the person asking the question gets their answer and I've learnt something new, my employer gains more recognition having a knowledgable employee, the client is more satisfied with a staff member with more expert knowledge. We all win...
    Last edited by grjohnson; 03-03-2003 at 07:19 PM.
    OCP 8i, 9i DBA
    Brisbane Australia

  9. #9
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    Well said grjohnson. The ability to research and obtain the answer is essential to the success of anyone that is technical (Not only a DBA).
    alapps

    Fast, Cheap, Reliable... Pick Two(2)

  10. #10
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    Regarding

    2)when you have row migration ?

    A row is placed in datablock initially and if it gets updated lateron,
    then the rowlength might increase.

    When it tries to expand,and if the space reserved for pctfree (For updates pctfree is reserved) has also been consumed.

    In that case Oracle will migrate the entire row to another block where it can fit that row

    And it maintains pointer to the new block containing the migrated row.

    Generally performance issue comesup if row is migrated /chained as
    it has to scan more than 1 block for getting that row.
    K.Chithra
    Oracle DBA

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