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System Change Number (SCN)...
SCN is a unique number assigned to every transaction
- A “transaction id”
- Critical to understanding reasons for recovery
SCN attached to every statement in transaction
- Put in rollback segment and redo log entries
Highest SCN recorded at checkpoint in:
- All datafile Headers
- Control File
Notes:
A System Change Number is maintained within the datafiles and control file to insure that the datafiles are up to date. A SCN is incremented and assigned to every transaction. The SCN is recorded in the datafile header block and the control file at log switch time (I.e. the "checkpoint SCN").