[DBMS] Navathe 21.24 Schedule classification

Question

Consider schedules S3, S4, and S5 below. Determine whether each schedule is
strict, cascadeless, recoverable, or nonrecoverable. (Determine the strictest
recoverability condition that each schedule satisfies.)
S3: r1 (X); r2 (Z); r1 (Z); r3 (X); r3 (Y); w1 (X); c1; w3 (Y); c3; r2 (Y); w2 (Z);
w2 (Y); c2;
S4: r1 (X); r2 (Z); r1 (Z); r3 (X); r3 (Y); w1 (X); w3 (Y); r2 (Y); w2 (Z); w2 (Y); c1;
c2; c3;
S5: r1 (X); r2 (Z); r3 (X); r1 (Z); r2 (Y); r3 (Y); w1 (X); c1; w2 (Z); w3 (Y); w2 (Y);
c3; c2;

Answer 1

Recoverable Schedule: The transaction which does uncommitted read operation should not commit before the commit/rollback of the transaction which updated that data item.
Non-recoverable Schedule: Opposite of recoverable schedule
Cascadeless Schedule: No uncommitted read is allowed.
Strict Recoverable Schedule: Read/Write(WR,WW) operations are not allowed by any other transactions until the transaction commit/rollback which updated a data item.

S3: recoverable, cascadeless, strict recoverable
S4: there is a dirty read between W3(Y) and R2(Y) hence not cascadeless, Not recoverable because T2 is committing before T3
S5: No dirty read hence recoverable and cascadeless shecdule but not strict recoverable because WW exists between W3(Y) and W2(Y)

Comments

Thanks, absolutely correct explanation.

---

In S5 schedule W2(Y) is written in T1 it should be written in T2 and now T2 is writing from committed T3 hence there is no WW operation hence it is strict recoverable.

@Deepak Poonia sir please look into this once.

@Manu Thakur