GATE CS 2024 Set 1 — Question 46
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Databases → Transactions & Concurrency → Conflict Serializability
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Question
Consider the following read-write schedule over three transactions , and , where the subscripts in the schedule indicate transaction IDs:Which of the following transaction schedules is/are conflict equivalent to ?
Correct answer
(B) T₁ T₃ T₂; (C) T₃ T₂ T₁; (D) T₃ T₁ T₂
Solution
To determine which serial schedules are conflict equivalent to , we construct the precedence graph (serialization graph).1. Identify Conflicts:
A conflict exists if two operations belong to different transactions, access the same data item, and at least one of them is a write operation.
A serial schedule is conflict equivalent to if it is a topological sort of the precedence graph.
A conflict exists if two operations belong to different transactions, access the same data item, and at least one of them is a write operation.
- Data item : Accessed only by (). No conflicts with other transactions.
- Data item : Accessed only by (). No conflicts with other transactions.
- Data item : Accessed by () and ().
- occurs before : Conflict (Write-Read).
- occurs before : Conflict (Write-Write).
- occurs before : Conflict (Read-Write).
- Nodes:
- Edges:
- has no incoming or outgoing edges (it is isolated).
A serial schedule is conflict equivalent to if it is a topological sort of the precedence graph.
- The constraint is must precede .
- can be placed anywhere relative to and .
1. (Option B)
2. (Option D)
3. (Option C)
4. Check Options:- (A) : Here precedes , which violates . Incorrect.
- (B) : precedes . Correct.
- (C) : precedes . Correct.
- (D) : precedes . Correct.
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