DBMS · Question 29 · Hard
DBMS question 29
Consider the schedule S: w1(X), r2(X), w2(Y), c2, c1, where ri, wi and ci are read, write and commit by transaction Ti. Which statement is correct?
- S is recoverable and cascadeless
- S is recoverable but not cascadeless
- S is not recoverable, because T2 read a value written by T1 and committed before T1 committed
- S is strict, because every transaction commits
Answer and explanation
Answer: C. S is not recoverable, because T2 read a value written by T1 and committed before T1 committed
Explanation: T2 reads X written by the uncommitted T1. If T1 later aborts, T2 has already committed using a value that never existed, and it cannot be undone. A recoverable schedule needs c1 before c2.
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