Redo log files are generally used for instance recovery and media recovery. The data needed for such operations is automatically recorded in the redo log files. However, a redo-based application may require that additional columns be logged in the redo log files. The process of logging these additional columns is called supplemental logging.
By default, Oracle Database does not provide any supplemental logging, which means that by default LogMiner is not usable. Therefore, you must enable at least minimal supplemental logging before generating log files which will be analyzed by LogMiner.
The following are examples of situations in which additional columns may be needed:
An application that applies reconstructed SQL statements to a different database must identify the update statement by a set of columns that uniquely identify the row (for example, a primary key), not by the ROWID shown in the reconstructed SQL returned by the V$LOGMNR_CONTENTS view, because the ROWID of one database will be different and therefore meaningless in another database.
An application may require that the before-image of the whole row be logged, not just the modified columns, so that tracking of row changes is more efficient.
A supplemental log group is the set of additional columns to be logged when supplemental logging is enabled. There are two types of supplemental log groups that determine when columns in the log group are logged:
Unconditional supplemental log groups: The before-images of specified columns are logged any time a row is updated, regardless of whether the update affected any of the specified columns. This is sometimes referred to as an ALWAYS log group.
Conditional supplemental log groups: The before-images of all specified columns are logged only if at least one of the columns in the log group is updated.
Supplemental log groups can be system-generated or user-defined.
In addition to the two types of supplemental logging, there are two levels of supplemental logging, as described in the following sections: