UGC NET CSE | December 2018 | Part 2 | Question: 83

Question

​​​​​​​Consider the following sequence of two transactions on a bank account $(A)$ with initial balance $20,000$ that transfers $5,000$ to another account $(B)$ and then apply $10 \%$ interest.

1. T1 start
2. T1 A old=$20,000$ new $15,000$
3. T1 B old = $12,000$ new = $17,000$
4. T1 commit
5. T2 start
6. T2 A old = $15,000$ new = $16,500$
7. T2 commit

Suppose the database system crashes just before log record (vii) is written. When the system is restarted, which one statement is true of the recovery process?

• A. We must redo log record (vi) to set $A$ to $16,500$
• B. We must redo log record (vi) to set $A$ to $16,500$ and then redo log records (ii) and (iii)
• C. We need not redo log records (ii) and (iii) because transaction $T1$ is committed
• D. We can apply redo and undo operations in arbitrary order because they are idempotent

Answer 1

As, there are no checkpoints. So, we will redo all the committed transactions and redo all the uncommitted transactions.

So, we will redo log record $(vi)$ and then redo log records $(ii)$ and $(iii)$

So, $(B)$ should be the answer.

Correct me if I'm wrong

For reference see here

Comments

we must redo step 3 and 4 , undo step 6 . None of the options are matching.

https://gateoverflow.in/981/gate2006-20-isro2015-17

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Option D should be the answer.

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given options are wrong

because correct answer is that we must undo step 6 and redo step 2 and 3.

this is the case of immidiate database modifications