GATE CS 2021 Set 1 — Question 42
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Databases → Transactions & Concurrency → Conflict Serializability
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Question
Let and denote read and write operations respectively on a data item by a transaction . Consider the following two schedules.Which one of the following options is correct?
Correct answer
(B) S₁ is not conflict serializable, and S₂ is conflict serializable.
Solution
A schedule is conflict serializable if its precedence graph (also known as serializability graph) is acyclic. The precedence graph contains a node for each transaction. An edge is drawn from transaction to () if an operation in conflicts with a subsequent operation in . Two operations conflict if they are on the same data item, belong to different transactions, and at least one of them is a write operation.Analysis of Schedule :
Let's find the conflicting pairs of operations:
Let's find the conflicting pairs of operations:
Let's find the conflicting pairs of operations:
1. and : comes before . This creates an edge .
2. and : comes before . This creates an edge .
The precedence graph for has a cycle: . Therefore, is not conflict serializable.Analysis of Schedule :Let's find the conflicting pairs of operations:
1. and : comes before . This creates an edge .
2. and : comes before . This creates an edge .
The precedence graph for has only one directed edge . Since the graph is acyclic, is conflict serializable. It is equivalent to the serial schedule .Based on the analysis, is not conflict serializable, and is conflict serializable. This corresponds to option (B).Continue learning with Success Tracker
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