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In database theory, what is the difference between "conflict serializable" and "conflict equivalent"?
My textbook has a section on conflict serializable but glosses over conflict equivalence. These are probably both concepts I am familiar with, but I am not familiar with the terminology, so I am looking for an explanation.
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Two schedules are conflict equivalent if the relative order of any two conflicting operations is the same in both schedules.
Conflict serializability is a subset of serializability. If a schedule is conflict serializable, it is implied that this schedules is serializable. It is computationally easier to determine if something is conflict serializable as opposed to just serializable. Simply construct a precedence graph.
Conflict Serializable: A schedule is called conflict serializable if it can be transformed into a serial schedule by swapping non-conflicting operations. Conflicting operations: Two operations are said to be conflicting if all conditions satisfy: They belong to different transactions. They operate on the same data item.
Conflict serializability orders any conflicting operations in the same way as some serial execution. A pair of operations is said to conflict if they operate on the same data item and one of them is a write operation. That means.
Conflict in DBMS can be defined as two or more different transactions accessing the same variable and atleast one of them is a write operation.
For example:
T1: Read(X) T2: Read (X) In this case there's no conflict because both transactions are performing just read operations.
But in the following case:
T1: Read(X) T2: Write(X) there's a conflict.
Lets say we have a schedule S, and we can reorder the instructions in them. and create 2 more schedules S1 and S2.
Conflict equivalent: Refers to the schedules S1 and S2 where they maintain the ordering of the conflicting instructions in both of the schedules. For example, if T1 has to read X before T2 writes X in S1, then it should be the same in S2 also. (Ordering should be maintained only for the conflicting operations).
Conflict Serializability: S is said to be conflict serializable if it is conflict equivalent to a serial schedule (i.e., where the transactions are executed one after the other).
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