Consider the following database schedule with two transactions, T1 and T2.
S = r2(X); r1(X); r2(Y); w1(X); r1(Y); w2(X); a1; a2;
where ri(Z) denotes a read operation by transaction Ti on a variable Z, wi(Z) denotes a write operation by Ti on a variable Z and ai denotes an abort by transaction Ti .
Which one of the following statements about the above schedule is TRUE?
(A) S is non-recoverable
(B) S is recoverable, but has a cascading abort
(C) S does not have a cascading abort
(D) S is strict
Answer: (C)
Explanation:
As we can see in figure,
- T2 overwrites a value that T1 writes
- T1 aborts: its “remembered” values are restored.
- Cascading Abort could have arised if – > Abort of T1 required abort of T2 but as T2 is already aborted , its not a cascade abort. Therefore, Option C
Option A – is not true because the given schedule is recoverable
Option B – is not true as it is recoverable and avoid cascading aborts;
Option D – is not true because T2 is also doing abort operation after T1 does, so NOT strict.
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