GATE CS 2015 Set 3 — Question 51
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Databases → Transactions & Concurrency → Recoverability
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Question
Consider the following partial Schedule involving two transactions and . Only the read and the write operations have been shown. The read operation on data item is denoted by read(P) and the write operation on data item is denoted by write(P).
Suppose that the transaction fails immediately after time instance 9. Which one of the following statements is correct?
| Time instance | T1 | T2 |
|---|---|---|
| 1 | read(A) | |
| 2 | write(A) | |
| 3 | read(C) | |
| 4 | write(C) | |
| 5 | read(B) | |
| 6 | write(B) | |
| 7 | read(A) | |
| 8 | commit | |
| 9 | read(B) |
Correct answer
(B) Schedule S is non-recoverable and cannot ensure transaction atomicity
Solution
S1 Analysis:
- is initialized to .
- Segment 1 computes . Value added is .
- Segment 2 computes . Value added is .
- Since , the final contents of Y are identical (). S1 is True.
- Segment 1 accesses .
- In C, 2D arrays are stored in row-major order. Elements of the same row are stored contiguously. Thus, and are adjacent in memory. S2 is True.
- Segment 2 accesses .
- These elements belong to the first column of different rows. In row-major order, they are separated by the size of a row ( elements). They are not contiguous. S3 is False.
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