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Re: Reading a comitted data before DBW writes it to the disk.

From: Somu <soumya_ongc_at_yahoo.com>
Date: 2 Apr 2004 22:07:32 -0800
Message-ID: <9666da71.0404022207.210635d8@posting.google.com>


"Michel Cadot" <micadot{at}altern{dot}org> wrote in message news:<406d69b9$0$12801$626a14ce_at_news.free.fr>...
> "Somu" <soumya_ongc_at_yahoo.com> a écrit dans le message de
> news:9666da71.0404020103.7e174d21_at_posting.google.com...
> > Hi,
> > I have some doubts on the above mentioned topic.
> >
> > As I understand, Oracle use Data buffer cache to store the data frm
> > data file in memory for efficient transaction. Consider a case, when a
> > row of a table is modified by a user,that means earlier row values are
> > in roll back segment,current changed value in data buffer cache and
> > info regarding this transaction in redo log buffer.Now suppose the
> > user has committed the transaction.So redo log buffer content gets
> > copied to redo log file.But as DBW writes to the data files at some
> > time interval(viz 3 sec) and commit itself doesnot invoke any
> > checkpoint so its clear that at this instance data files are not
> > updated.Now suppose the user issues Select query to see that
> > particular row fields ,now as par with data consistency Oracle will
> > show the comitted data.
> > Now my question comes into picture.Where from Oracle
> > show this updated data? Apparently it seems from Data Buffer cache.So
> > does this cache has information that explicitly says following info
> > a)this block is changed but not comitted
> > b)this block is changed and comitted but not written back
> > to data files
> > c)this block is changed and comitted and written back to
> > data files
> >
> > Waiting for some positive response.
> > Thanks and Regards
> > somu
>
> Have a look at Metalink note 40689 "ORA-01555 "Snapshot too old" - Detailed Explanation", url:
> http://metalink.oracle.com/metalink/plsql/ml2_documents.showDocument?p_database_id=NOT&p_id=40689.1
>
> There is a detailed description of how "commited" flag is propagated.
>
> Regards
> Michel Cadot

Michel

Thanks for your reponse.But unfortunately I cant open your link.I dont have the required access to this metalink site. Rgds
Somu Received on Sat Apr 03 2004 - 00:07:32 CST

Original text of this message

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