Equivalence of Schedules | Equivalent Schedules in DBMS

Equivalence of Schedules-

 

In DBMS, schedules may have the following three different kinds of equivalence relations among them-

 

 

1. Result Equivalence
2. Conflict Equivalence
3. View Equivalence

 

1. Result Equivalent Schedules-

 

• If any two schedules generate the same result after their execution, then they are called as result equivalent schedules.
• This equivalence relation is considered of least significance.
• This is because some schedules might produce same results for some set of values and different results for some other set of values.

 

2. Conflict Equivalent Schedules-

 

If any two schedules satisfy the following two conditions, then they are called as conflict equivalent schedules-

1. The set of transactions present in both the schedules is same.
2. The order of pairs of conflicting operations of both the schedules is same.

 

3. View Equivalent Schedules-

 

We have already discussed about View Equivalent Schedules.

 

PRACTICE PROBLEMS BASED ON EQUIVALENCE OF SCHEDULES-

 

Problem-01:

 

Are the following three schedules result equivalent?

 

 

Solution-

 

To check whether the given schedules are result equivalent or not,

• We will consider some arbitrary values of X and Y.
• Then, we will compare the results produced by each schedule.
• Those schedules which produce the same results will be result equivalent.

 

Let X = 2 and Y = 5.

On substituting these values, the results produced by each schedule are-

 

Results by Schedule S1- X = 21 and Y = 10

Results by Schedule S2- X = 21 and Y = 10

Results by Schedule S3- X = 11 and Y = 10

 

• Clearly, the results produced by schedules S1 and S2 are same.
• Thus, we conclude that S1 and S2 are result equivalent schedules.

 

Problem-02:

 

Are the following two schedules conflict equivalent?

 

 

Solution-

 

To check whether the given schedules are conflict equivalent or not,

• We will write their order of pairs of conflicting operations.
• Then, we will compare the order of both the schedules.
• If both the schedules are found to have the same order, then they will be conflict equivalent.

 

For schedule S1-

 

The required order is-

• R₁(A) , W₂(A)
• W₁(A) , R₂(A)
• W₁(A) , W₂(A)

 

For schedule S2-

 

The required order is-

• R₁(A) , W₂(A)
• W₁(A) , R₂(A)
• W₁(A) , W₂(A)

 

• Clearly, both the given schedules have the same order.
• Thus, we conclude that S1 and S2 are conflict equivalent schedules.

 

Next Article- Introduction to Relational Algebra

 

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