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GATE CSE 2021 Set 2 Q42: Conflict serializability and recoverability of a schedule

Question

Let S be the following schedule of operations of three transactions T1T_1, T2T_2 and T3T_3 in a relational database system:

R2(Y),R1(X),R3(Z),R1(Y),W1(X),R2(Z),W2(Y),R3(X),W3(Z)R_2(Y), R_1(X), R_3(Z), R_1(Y), W_1(X), R_2(Z), W_2(Y), R_3(X), W_3(Z)

Consider the statements P and Q below:

P: S is conflict-serializable.

Q: If T3T_3 commits before T1T_1 finishes, then S is recoverable.

Which one of the following choices is correct?

Try it: multiple choice: exactly one correct option

Select one option
Wrong answer costs 2/3 mark in GATE.
Show answer and explanation

Answer: B (official GATE 2021 final answer key)

Concept used: Precedence graph for conflict serializability; recoverability requires a reader to commit after the writer it read from.

First step: List conflicting pairs: R1(Y)R_1(Y) before W2(Y)W_2(Y), W1(X)W_1(X) before R3(X)R_3(X), R2(Z)R_2(Z) before W3(Z)W_3(Z); then note that T3T_3 reads X written by T1T_1.

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Source: official GATE 2021 Set 2 CS question paper and final answer key, organised by IIT Bombay. Question paper · Answer key

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