SCN

The SCN (System Change Number) is Oracle’s internal counter that orders every change made to the database. It grows strictly monotonically: each COMMIT receives a higher SCN than the previous one, making it possible to reconstruct the exact state of the database at any past point in time.

How it works #

Whenever a transaction issues a COMMIT, Oracle assigns a unique SCN and records it in the redo log, the control file, and the datafile headers. This value is the reference point for all consistency and recovery operations.

-- Read the current database SCN
SELECT CURRENT_SCN FROM V$DATABASE;

-- SCN recorded in a datafile header
SELECT NAME, CHECKPOINT_CHANGE# FROM V$DATAFILE;

During instance recovery, Oracle compares the SCN stored in the control file against the SCN in each datafile header to determine which blocks require redo and which are already consistent.

Operational context #

The SCN is central to three main scenarios:

• Point-in-time recovery (PITR): a target SCN is specified and Oracle reapplies redo up to that exact point.
• Flashback: Flashback Query and Flashback Database use the SCN to navigate through data history.
• Data Guard and replication: the standby applies archived redo up to the SCN transmitted by the primary, ensuring synchronization.

The SCN has a theoretical maximum (tied to the 48-bit architecture in recent versions), but under normal conditions this is not an operational constraint. Abnormal situations of reduced SCN headroom can be monitored via V$DATABASE_INCARNATION and related MOS notes.