Multiple choices questions in DBMS, Which is non-recoverable schedule? How do we find whether a schedule is recoverable or not? Can we permit a transaction that has used a data item written by another uncommitted transaction?

DBMS MCQ - Non-recoverable schedules

1. In a schedule S with two transactions T1 and T2, T1 reads the data item which was produced by T2 and T1 commits before T2 commits. In this case, the schedule S is said to be a _________ schedule.

a) recoverable

b) non-recoverable

c) conflict serializable

d) serial

Answer: (b) non-recoverable

The given case is a non-recoverable schedule. Let us suppose that the transaction T2 decided to rollback after T1 commits. In this case, the value of the data item consumed by T1 was wrong. We have to rollback T1 which cannot be done because T1 has committed. As per the durability property, the rollback is not permitted for a committed transaction.

How to avoid non-recoverable schedules?

To avoid such a situation, we can postpone the commit of T1 until T2 commits.

Recoverable schedule.

Let us consider a schedule with two transactions T1 and T2. In this schedule, if the transaction T2 reads the value of data item x which was produced by T1, then the commit of T1 must happen before that of T2. Such a schedule is recoverable schedule.

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