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Conflict-serializable schedules: conflict-serializability

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• Conflict-equivalent schedules
  • Conflict-equivalent schedules:

    2 schedules S1 and S2 are conflict equivalent iff Schedule S1 can be transformed into schedule S1 by a sequence of non-conflicting swaps of adjacent actions

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  • Example:

    Consider the following 2 schedules: The schedules are conflict equivalent because:

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• Conflict-serializability
  • Conflict-serializable schedules:

    A schedule S1 is conflict serializable iff The schedule S1 can be transformed into a serial sechdule by a sequence of non-conflicting swaps of adjacent actions

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  • Example:

    Consider the following schedule: We perform the following sequence of non-conflicting swaps: and obtain a serial schedule !!!

    Conclusion:

    The schedule: is a conflict-serializable schedule

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• Conflict-serializable schedule are always serializable
  • Fact:

    A conflict-serializable schedule is (always) a serializable schedule

    Reason:

    Swapping two non-conflicting operations in a schedule will not change the effect of the execution of the operations in a schedule Therefore: The effect of the execution of a conflict-serializable schedule is equivalent to the execution of a serial schedule of the transactions for every database state !!! So by definition of a serializable schedule: (see: click here) A conflict-serializable schedule is a serializable schedule

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• Schedules can be serializable but not conflict-serializable
  • Example of a schedule that is serializable but non-conflict serializable:

    Consider the following 3 transactions: Consider the following 2 schedules with the 3 transactions: Schedule S2 is a serial schedule !!! The effect (= state of the database) of schedules S1 and S2 are identical: (The database state = the values of the database elements) Therefore: The schedule S1 is a serializable schedule !!! because (by definition): Schedule S1 is equivalent to the serial schedule S1 for every database state !!! However: We cannot transform the schedule S1 into a serial schedule by swapping adjacent non-conflicting actions: Therefore, by definition: The schedule S1 is not a conflict-serializable schedule !!!