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log-based recovery Recovery: Assuming a redo-only recovery, what should the database do to recover if the...

log-based recovery

Recovery: Assuming a redo-only recovery, what should the database do to recover if the following is the contents of the log upon recovery:

• T1 write X: was 3, now 6

• T2 write Y: was 2, now 0

• T1 commit

• T3 write X: was 6, now 4

• T2 commit

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Answer #1

T1 write X: was 3, now 6
T2 write Y: was 2, now 0
T1 commit
T3 write X: was 6, now 4
T2 commit

Transaction Ti needs to be redone if log contains record Ti start and either the record Ti commit or the record Ti abort.
In above Log we can see that Transaction T1 and T2 commit before System crash.
Therefore, Transaction T1 and T2 will be Redone.

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