Serializability

Question

My doubt is ... when checking serializbility why do we check conflict and view serializability ..i mean result equivalence would be better ... no doubt it would be more complex but if any transaction is giving the same result as serial schedule then its ok ... secondly can there be a schedule  which has different order of conflicting operations but same result ?? in that case would it be equivalent to serial schedule ??

Answer 1

1] Why not use result equivalence?

$\rightarrow$ Serial schedules need not be result equivalent.

Consider this simple example with 2 transaction T₁ and T₂ where x = 100 initially;

S₁ : T₁(x=x+100);T₂(x=x/2) Result₁ : x=100

S₂ : T₂(x=x/2);T₁(x=x+100) Result₂ : x=150

$\rightarrow$ Now if we go with result equivalence then we have to compare result of concurrent schedule with every possible serial schedule. This is hell of a computation.

Also, there is redundency in calculation. I mean, consider the fact that data item (x here) will have dynamic value. As value of x is not fixed, we will have to calculate result of every serial schedule everytime and compare it with concurrent schedule. Now, as we are computing every serial schedule why do it concurrently again??!

2] Can there be a schedule which has different order of conflicting operations but same result ?

$\rightarrow$ Yes, there is a possibility of this. (You think of the example ;-) )

It would be called as result equivalent but not conflict equivalent.

Comments

thank you so much !!