Multiple choice

Consider the following log sequence of two transactions on a bank account, with initial balance of Rs. 12,000, that transfers 2000 to a mortgage payment and then applies 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 statement is true for the recovery procedure?

  1. We must redo log record 6 to set B to 10500.

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

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

  4. We can apply redo and undo operations in arbitrary order because they are idempotent.
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C Correct answer
Explanation

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