When you create a Replication Agent instance, you can choose to enable supplemental logging of primary key (PK) and unique index (UI) columns at the table level.
Supplemental logging at database level is not essential for Replication Agent while replicating. Instead, you can enable supplemental logging of PK and UI columns at table level for the Oracle system tables and user tables that need to be replicated. After you enable supplemental logging of PK and UI columns at table level, Oracle only writes those columns of PK and UI to the redo log file, whenever the table is updated. This reduces the size of redo log file compared to supplemental logging of PK and UI at database level. For a table whose supplemental logging is not enabled, Oracle writes columns of PK and UI to the redo log file, only when any column of PK or UI is updated.
You can switch between supplemental logging at database level and table level at any point in time. Do not turn off supplemental logging at database level before you turn on supplemental logging at table level because it takes some amount of time for switching. During this period, Oracle does not write the PK and UI columns to the redo log file, until the supplemental logging at table level is turned on. This can cause Replication Agent to go to the Replication Down state.
See the Replication Agent 15.7.1 SP100 Administration Guide and the Replication Agent 15.7.1 SP100 Reference Manual.