[DBMS] Lost Update and WW

Question

Consider thw following schedules :-

S1 :- W1(A) W2(A) W1(B) W2(B)

S2:- W1(A) R2(A) W2(A) W2(B) W1(B)

Assume that schedule S1 is failed after performing W1(B). Which of the following options are true?

a.) S1 and S2 has lost update problem and WW problem.

b:) S1 and S2 are free from lost update problem and WW problem

c) S1 has WW problem and S2 has lost update problem

d) S1 has lost update problem and s2 has WW problem

Answer 1

S1 has lost update problem and S2 has WW and WR problems. So, d) seems correct.

Comments

Please share the reason for the same. In my opinion both are the same thing.

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Since S1 fails after performing W1(B), its value is lost and overwritten by T2 after performing W2(B).

For better understanding refer this post https://gateoverflow.in/3862/please-explain-example-problem-problem-difference-between

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But there Arjun sir mentioned that they are same.In s2,t2 does W(A) followed by t1 W(A),so the update done by t2 is also lost here.

Even in korth it says 

The Lost Update Problem. This problem occurs when two transactions that
access the same database items have their operations interleaved in a way that makes
the value of some database items incorrect.Suppose that transactions T1 and T2 are
submitted at approximately the same time, and suppose that their operations are
interleaved as shown in Figure 21.3(a); then the final value of item X is incorrect
because T2 reads the value of X before T1 changes it in the database, and hence the
updated value resulting from T1 is lost.

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Yes, Lost update or W-W conflict both are same thing.

similary Dirty-read and W-R also.

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So what will be answer @Reena_kandari?

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S2 has lost update, because of W2(B) -W1(B)

and in S1, data written by T1 is lost due to updation done by T2  but later T1 itself fails ,so we don't need to consider its lost update, So I think in S1 there is no lost update..

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1. In s2 W1(A) W(A) ,W2(B) W1(B) both are causing lost updates or only W2(B) - W1(B)?

2. In S1 we have lost updates of T1 so it should be Lost update problem?

Please correct me if wrong

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No, In s2 only W2(B) -W1(B) cause Lost update problem.insted of first reading it is directly writing the value or we can say blind write. first read and then write is normal operations.

I think in S1 also there is loss update, what is the answer?

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I dont have the answer. In s2 even if it is not the blind write,but still update of T1 on A is getting lost by T2 update on A,so why it is not a lost update? Lost update means simply some updated value is lost what i think .?

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for your doubt https://www.youtube.com/watch?v=N7N-TdqNVRc

but I am not sure whether S1 has lost update or not, because S1 is not going to commit the data so its update at the end is going to lost irrespective of any W-W conflict.

@Bikram sir please guide 

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@reena_kandari

Is lost update problem possible in serializable schedule?