GATE | GATE CS 2021 | Set 2 | Question 42

Let S be the following schedule of operations of three transactions T1, T2 and T3 in a relational database system:

R2(Y),R1(X),R3(Z),R1(Y)W1(X),R2(Z),W2(Y),R3(X),W3(Z) 

Consider the statements P and Q below:

• P: S is conflict-serializable.
• Q: If T3 commits before T1 finishes, then S is recoverable.

Which one of the following choices is correct?
(A) Both P and Q are true
(B) P is true and Q is false
(C) P is false and Q is true
(D) Both P and Q are false


Answer: (B)

Explanation:

T1	T2	T3
	R(Y)
R(X)
		R(Z)
R(Y)
W(X)
	R(Z)
	W(Y)
		R(X)
		W(Z)

Hence, Statement P is conflict serializable.

Schedule S is recoverable, if T_j creating the dirty read by reading the written data by T_i and T_j commits after T_i commits.

T1 and T2 must be commit before T3 as T3 dirty reads the value at T1 and T2, so if T3 commits before T1 finishes, then S will not recoverable.

Hence Statement Q is false. 

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