GATE | GATE-CS-2009 | Question 43

Consider two transactions T1 and T2, and four schedules S1, S2, S3, S4 of T1 and T2 as given below:

T1 = R1[X] W1[X] W1[Y]
T2 = R2[X] R2[Y] W2[Y]
S1 = R1[X] R2[X] R2[Y] W1[X] W1[Y] W2[Y]
S2 = R1[X] R2[X] R2[Y] W1[X] W2[Y] W1[Y]
S3 = R1[X] W1[X] R2[X] W1[Y] R2[Y] W2[Y]
S1 = R1[X] R2[Y]R2[X]W1[X] W1[Y] W2[Y]

Which of the above schedules are conflict-serializable?

(A) S1 and S2
(B) S2 and S3
(C) S3 only
(D) S4 only

Answer: (B)

Explanation: There can be two possible serial schedules T1 T2 and T2 T1. The serial schedule T1 T2 has the following sequence of operations
R1[X] W1[X] W1[Y] R2[X] R2[Y] W2[Y]
And the schedule T2 T1 has the following sequence of operations.
R2[X] R2[Y] W2[Y] R1[X] W1[X] W1[Y]
The Schedule S2 is conflict-equivalent to T2 T1 and S3 is conflict-equivalent to T1 T2

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