$\color{red}\text{Correct option is D but it is not what you think.}$
$\color{red}\text{Just a slight modification in the given schedule}$
$\color{red}\text{will change the answer of this question.}$
$\color{red}\text{Debunking a Major Misconception:}$
Majority of aspirants has misconception that serializability is same as view serializability. They think view serializability is necessary and sufficient for serializability. But it is not so. You may potentially loose marks in future GATE Exam if your concepts are not correct. To justify this, I'll give counter example (just a slight variation of this question) at the end.
Since computations are also given along with read and write operations, just checking view serializability is not enough to conclude that given schedule is not serializable. Because every view serializable schedule is serializable but not every serializable schedule is view serializable.
From NPTEL:
It may so happen that given schedule is neither CS nor VS but serializable just because of detailed behaviour of transactions are given. Its just coincidence the wrong answers posted here are getting correct option. Even faulty clock shows correct time twice a day.
General Serializability definition from Ullman:
(Based on above definiton, we'll analyze the GATE question.)
From Ullman:
In above example, given schedule is neither CS nor VS but it is serializable. Its equivalent to serial schedule $(T1,T2)$ and $(T2,T1)$ both. If this example had been given in the question, then the answer would have been option $C$.
Now, coming to the given question.
We have to check if the effect (final database state) of given schedule in question is equal to effect of SOME serial schedule (where transactions execute one after another without interleaving)..
Assume that initially $A=x$ and $B=y$. Effect of given schedule -
If we execute above schedule, then final value of $A$ and $B$ will be $x-10$ and $y+0.2x$ respectively.
If there are $n$ transactions in schedule then $n!$ serial schedules are possible. Since there are two transactions involved in given schedule, only $2!(=2)$ serial schedules are possible. Now, Let's check effect of both serial schedule.
Effect of serial schedule $S_1 : (T1, T2)$ -
Effect of serial schedule $S_2 : (T2,T1)$ -
We can see that effect of given schedule is neither equal to effect of $S_1$ nor equal to effect of $S_2$.
So, given schedule is neither serializable as $T1, T2$ nor as $T2, T1$. Option D is correct.
Now consider the following schedule $S_{t}$ which is variation of given question, where I replaced $Temp = 0.2*A$ with $Temp = A$ :
Effect of $S_t$ -
Effect of serial schedule $S_{t1} : (T1,T2)$ -
Effect of serial schedule $S_{t2} : (T2, T1)$ -
We can easily see that for every initial database state, effect of $S_t$ is equal to effect of serial schedule $S_{t1} : (T1, T2)$.
So, schedule $S_t$ is serializable only as $T1,T2$. Here, option A is correct, not option D.
But $S_t$ is neither view serializable nor conflict serializable even though $S_t$ is serializable.
Note:
- When computations are given in the Schedule, we have to check for general serializability.
- When computations are not given and only Read and Write operations are given then in that case, Serializability $\equiv$ View Serializability.
- When computations and blind writes are not there, Serializability $\equiv$ View Serializability $\equiv$ Conflict Serializability.
