When you make a DML change to a row, Oracle writes a change vector to the Log Buffer. This change vector includes the before and after images of the affected parts of that row. If you updated one column, the change vector includes the before and after values of that column. If you inserted a new row, the Log Buffer only contains the new information and there is no old information to include. If you delete a row, the Log Buffer includes the entire row as it existed before the delete, along with a command to delete the row.
Eventually, this information is dumped to the online redo logs. If you are running in archivelog mode, then the information in the online redo logs will be copied to the archivelog destination.
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