Q. 35 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.
- T1 start
- T1 B old = 12000 new = 10000
- T1 M old = 0 ne = 2000
- T1 commit
- T2 start
- T2 B old = 10000 new = 10500
- T2 commit
Suppose the database system crashed just before log record 7 is written. When the system is restarted, which one statement is true of the recovery procedure?
(A) We must redo log record 6 to set B to 10500
(B) We must undo log record 6 to set B to 10000 and then redo log records 2 and 3
(C) We need not redo log records 2 and 3 because transaction T1 has committed
(D) We can apply redo and undo operations in arbitrary order because they are Idempotent
Answer: (B)
Explanation:
