GATE CS 2020 Set 1 — Question 47
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Databases → Transactions & Concurrency → Conflict Serializability
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Question
Consider a schedule of transactions and :
Here, RX stands for "Read(X)" and WX stands for "Write(X)". Which one of the following schedules is conflict equivalent to the above schedule?
| RA | RC | WD | WB | Commit | ||||||
|---|---|---|---|---|---|---|---|---|---|---|
| RB | WB | RD | WC | Commit |
Correct answer
(A) T1 RA RC WD WB Commit ------ ------ ------ ------ ------ ------ ------ ------ ------ ------ ------ T2 RB WB RD WC Commit
Solution
To determine conflict equivalence, we must ensure that the order of all conflicting operations (operations on the same data item by different transactions where at least one is a write) is preserved.Original Schedule ():
Sequence of operations: .Conflicting Pairs in :
Sequence: .
Sequence: executes completely before starts conflicting operations.
Sequence:
Sequence: executes almost completely before .
Sequence of operations: .Conflicting Pairs in :
1.On B: vs Order:
2.On B: vs Order:
3.On C: vs Order:
4.On D: vs Order:
Analyze Option (A):Sequence: .
- On B: and (Preserved)
- On C: (Preserved)
- On D: (Preserved)
Sequence: executes completely before starts conflicting operations.
- On B: (Conflict reversed: Original was )
Sequence:
- On D: (Conflict reversed: Original was )
Sequence: executes almost completely before .
- On C: (Conflict reversed: Original was )
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