Re: supplemental logging questions and golden gate

"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@xxxxxxxxx>
To: ORACLE-L <oracle-l@xxxxxxxxxxxxx>
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?

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