What is the difference between "conflict serializable" and "conflict equivalent"?
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.
Answer
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).