Consider three data items D1, D2 and D3 and the following execution schedule of transactions T1, T2 and T3. In the diagram, R(D) and W(D) denote the actions reading and writing the data item D respectively.
Which of the following statements is correct?
(A) The schedule is serializable as T2; T3; T1
(B) The schedule is serializable as T2; T1; T3
(C) The schedule is serializable as T3; T2; T1
(D) The schedule is not serializable
Answer: (D)
Explanation:
T1 and T2 have conflicting operations between them forming a cycle in the precedence graph.
R(D2) of T2, and W(D2) of T1 ( Read-Write Conflict)
R(D1) of T1, and W(D1) of T2 ( Read-Write Conflict)
Hence in the precedence graph of the schedule there would be a cycle between T1 and T2 vertices. Therefore not a serializable schedule.
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