Consider the following two statements about database transaction schedules: I.…
2019
Consider the following two statements about database transaction schedules:
I. Strict two-phase locking protocol generates conflict serializable schedules that are also recoverable.
II. Timestamp-ordering concurrency control protocol with Thomas’ Write Rule can generate view serializable schedules that are not conflict serializable.
Which of the above statements is/are TRUE?
- A.
I only
- B.
II only
- C.
Both I and II
- D.
Neither I nor II
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Correct answer: C
Answer: Both statements are true.
Statement I — Strict two-phase locking (strict 2PL): Holding exclusive locks until a transaction commits or aborts prevents other transactions from reading or writing those items prematurely. This enforces conflict serializability and also ensures recoverability by preventing cascading aborts.
Statement II — Timestamp-ordering with Thomas' Write Rule: Thomas' rule allows skipping writes that are obsolete with respect to timestamps. This can produce schedules that are view serializable (final read/write relationships are preserved) but not conflict serializable, so the statement is true.
Therefore, both statements are correct.
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