Re: supplemental logging questions and golden gate
Date: Fri, 22 Jul 2011 10:08:35 -0400
Message-ID: <CAE-dsOJYeW344PhF4v0La8ysTq59hsfBwhO-rSQuQ2pYdZ1HYw_at_mail.gmail.com>
I am confused. I thought supplemental logging only needs to log the before image of every column, if you do not log based on a unique or primary key? If you do, it only gets the before image of the column that changed. However, that before image should already be logged to redo, since that image is copied to the undo tablespace and that generates redo.
On Tue, Jul 19, 2011 at 1:53 PM, David Fitzjarrell <oratune_at_yahoo.com>wrote:
> "However, isn't the before image already captured? " Only of the updated
> columns, iirc. Supplemental logging ensures that every supplementally
> logged column has its image captured before the update, either always
> (whether or not a supplemental column is updated) or conditionally (only
> when at least one of the supplemental columns is updated).
>
> The redo logs track the rowid and all updated column images; the addition
> of the pk ensures the correct records are processed across systems as ROWID
> values are not the same between two databases having the same tables,
> tablespaces and data.
>
> "not having a key on a table wth alot of columns can significantly increase
> redo generation." Yes, and no, depending upon which supplemental logging
> scheme you're using as likely you'll have all columns of a table
> supplementally logged. Recovery should be minimally impacted by this.
>
> David Fitzjarrell
>
>
> *From:* Dba DBA <oracledbaquestions_at_gmail.com>
> *To:* ORACLE-L <oracle-l_at_freelists.org>
> *Sent:* Tuesday, July 19, 2011 7:48 AM
> *Subject:* supplemental logging questions and golden gate
>
> We are going to use supplemental logging to work with Golden Gate.
> Golden Gate is going to be used to stream data to other Oracle Databases and
> to Teradata. I have some questions about supplemental loggings.
>
>
> http://download.oracle.com/docs/cd/B19306_01/server.102/b14215/logminer.htm#sthref2000
>
> 1. I believe the documentations says that during updates supplemental
> logging adds redo for 2 things
> -- primary or unique key (assuming we have a key and do not need to do
> all records)
> -- before image of the update
>
> However, isn't the before image already captured? Whern you do an update,
> the befoer image is copied to the undo tables space which is then written to
> redo. This is necessary for rollback. I think this is saying that the before
> image is needed after a commit is done, so that downstream Golden Gate can
> tell which data to apply? I am not sure why. If you have the key and the
> after image, isn't that enough? Or is this for some kind of rollback
> downstream?
>
> 2. Inserts/Deletes: just adds the primary or unique key (assuming you have
> one and do not add need to log all rows). I believe redo logs normally just
> track the rowid right?
> 3. not having a key on a table wth alot of columns can significantly
> increase redo generation. Since if you are updating 1 row, you need to write
> every column to the redo logs so that downstream golden gate can do a diff
> to apply the changes. Does this affect recovery at all other than you have
> more redo. Or will the racle recovery mechanism ignore the supplemental
> logging?
>
>
>
-- http://www.freelists.org/webpage/oracle-lReceived on Fri Jul 22 2011 - 09:08:35 CDT