An uncommitted transaction can be thought of as a transaction that never existed.... so on database resumption, it will be as if the transaction never started.

Of course, in reality, its likely that many database blocks (even on disk) were changed as part of that transaction, so when the database comes back online we need to sort out that mess.

*Every* change in the database, even the changes to the undo area to allow a rollback of a transaction are captured in the redo logs. Thus when the database starts, using the redo logs we can put the *undo* area back to the same state it was the moment the database crashed.

Inside that undo area is the information we need to undo any uncommitted transactions, so we go ahead and do that to make sure no mess is left lying around in the database.

Hope that helps.