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UGC-NET | UGC NET CS 2018 Dec – II | Question 59
  • Last Updated : 16 Sep, 2020

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.

(i) T1 start
(ii) T1 A old=20000 new 15,000
(iii) T1 B old=12000 new=17000
(iv) T1 commit
(v) T2 start
(vi) T2 A old=15000 new=16500
(vii) T2 commit 

Suppose the database system crashes out just before log record (vii) is written. When the system is restricted, which one statement is true of the recovery process ?
(A) We can apply redo and undo operation in arbitrary order because they are idempotent.
(B) We must redo log record (vi) to set A to 16,500.
(C) We must undo log record (vi) to set A to 16,500 and then redo log record (ii) and (iii).
(D) We need not redo records (ii) and (iii) because transaction T1 has committed.

Answer: (C)


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