Consider the following log sequence of two transactions on a bank account,…

Consider the following log sequence of two transactions on a bank account, with initial balance 12000, that transfer 2000 to a mortgage payment and then apply a 5% interest. 1. T1 start 2. T1 B old=1200 new=10000 3. T1 M old=0 new=2000 4. T1 commit 5. T2 start 6. T2 B old=10000 new=10500 7. T2 commit Suppose the database system crashes just before log record 7 is written. When the system is restarted, which one statement is true of the recovery procedure?

  1. A.

    We must redo log record 6 to set B to 10500

  2. B.

    We must undo log record 6 to set B to 10000 and then redo log records 2 and 3

  3. C.

    We need not redo log records 2 and 3 because transaction T1 has committed

  4. D.

    We can apply redo and undo operations in arbitrary order because they are idempotent.

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Correct answer: B

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