When you start the database after an abnormal end, the database will look at the datafiles to find the last time...
data was written to those datafiles. The database will then look at the online redo logs to see what transaction occurred (committed or uncommitted) after the database datafiles were last updated. Once the database knows what went on, it can replay the transactions that were not written to the database datafiles. This process is called "rolling forward." Some of the transactions that are not rolled forward were not committed, therefore, the database has to undo, or "roll back" those uncomitted transactions.
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