Undo 가 하는 일

1. Rollback an active transaction

2. Recover a terminated transaction

3. Provide read consistency Undo_Retention

MCVV(Multi-Version Concurrency Control )

# QUERY SCN >= BLOCK SCN ==> 그냥 Read

# QUERY SCN < BLOCK SCN ==> CR Operation 수행 ( UNDO Base 로 )

4. Recovery From Logical Corruption (=Flashback ) Undo_Retention