Log Based Changes – Hot Mining or Cold Mining

Many times, the Capture process is temporarily stopped or disabled while the transaction activity continues and log switching occurs as usual. When the Capture process resumes, the LogMiner session will go back to the point where it left off. It is possible that the starting point will be located in an archived log file.

When the transactional activity contents are extracted from the redo log files by the LogMiner Session for the Capture process, the process is called Hot Mining. When LogMining has to be done on the archived redo log files, it is called cold mining. This condition often occurs when the capture process falls back in time and frequent log switching takes place.

Supplemental Logging

Redo log files record all of the information needed for instance recovery. Now that redo log files and archived redo log files are being used for the likes of Oracle Streams and LogMiner, some extra columns of information need to be added to the redo log file. The process of logging extra columns to the redo log files is called Supplemental Logging. Thus is not a default behavior. It has to be enabled either at the database level or at the object level.

Why is this additional logging needed? When a particular column is updated at the source database table for a set of rows, the values in the column or columns are logged by default. When these values are moved to the destination side, to which rows does Oracle apply them, or how does Oracle identify the rows to be updated? Supplemental logging provides the answers to these questions. The additional column values are used by Oracle to identify the rows that are to receive the information.