Questions on Transactions

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Questions on Transactions
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• Question. Is the following schedule conflict
  serializable? Why?

3

• Answer. Equivalent serial schedule T2, T1, T3

4

• Question. Can the following schedule be adopted
  by a database?

5

• Answer. No, because it is not recoverable. Note
  that T1 reads a value of y updated by T2, and
  yet, T1 commits before T2.

6

• Question. Is the following schedule serializable?
  How about conflict serializable?

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• Answer. Is it conflict serializable?
• No.
• Is it serializable?
• Yes. The schedule is equivalent to serial
  schedule T1, T2, T3.
• Remember

8

• Question. If T2 aborts at the position shown,
  which transactions need to be rolled back?

9

• Answer. All of them.

10

• Question. Why must every schedule be serializable
  and recoverable?
• Why is a cascadeless schedule usually a good
  idea?
• Can you give a scenario where a cascadeless
  schedule is not a good idea?

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• Why must every schedule be serializable and
  recoverable?
• Serializability of a schedule makes the result
  predictable and natural it must be equivalent
  to one of the serial schedules.
• Recoverability ensures that, when failures
  happen, the database state can be brought back to
  a correct state.
• Why is a cascadeless schedule usually a good
  idea?
• Because it saves the computing resources which
  would be wasted if the transactions are rolled
  back.
• Can you give a scenario where a cascadeless
  schedule is not a good idea?
• A casecadeless schedule typically has a lower
  level of concurrency. If failures are infrequent,
  it would be a good idea to increase concurrency
  by taking the risk of cascading roll backs.