The GATE Grind

GATE 2025 CS (CS2) – Question 53

Databases · Transactions and Concurrency Control · 2 marks · Multiple select

Consider the database transactions T1 and T2, and data items X and Y. Which of the schedule(s) is/are conflict serializable?

Transaction T1: R1(X), W1(Y), R1(X), W1(X), COMMIT(T1)
Transaction T2: W2(X), W2(Y), COMMIT(T2)

  1. R1(X), W2(X), W1(Y), W2(Y), R1(X), W1(X), COMMIT(T2), COMMIT(T1)
  2. W2(X), R1(X), W2(Y), W1(Y), R1(X), COMMIT(T2), W1(X), COMMIT(T1)
  3. R1(X), W1(Y), W2(X), W2(Y), R1(X), W1(X), COMMIT(T1), COMMIT(T2)
  4. W2(X), R1(X), W1(Y), W2(Y), R1(X), COMMIT(T2), W1(X), COMMIT(T1)

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Show answer and explanation

Correct answer: (B) W2(X), R1(X), W2(Y), W1(Y), R1(X), COMMIT(T2), W1(X), COMMIT(T1)

Explanation

In B, every conflict is ordered T2 → T1, so the precedence graph is acyclic. In A both T1→T2 (R1(X) before W2(X)) and T2→T1 (W2(X) before the second R1(X)) occur. C and D also have edges in both directions, so they contain cycles.