Virtual Gate Test Series: Databases - Schedule

Question

Which of the following is/are true?

I. Every strict schedule is cascade less schedule and recoverable schedule

II. Every cascade less schedule is recoverable schedule

III. Every cascading rollback schedule is recoverable schedule

IV. Every cascading rollback schedule is a strict schedule

V. Every recoverable schedule is cascading rollback schedule

1. All of the above
2. I, II, and IV only
3. II, III, IV, and V only
4.  I, II, and III only

Comments

should be D?

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Hope it helps

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@Bhagyashree Mukherje

Cascading aborts or cascading rollback are same??

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I think no its different

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Abort: Also known as "rollback". Aborting a transaction consists of ensuring that state of the database becomes as if the transaction had never begun. Any changes made by the transaction must be undone and we must ensure that those changes had no effect on any concurrently executing transactions.

https://courses.cs.washington.edu/courses/cse444/00wi/glossary.html

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I think abort happens, for that rollback occurs

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Yes thanks for rectifying.Its the same.Just read Bikram sir's comment in this question.

https://gateoverflow.in/39590/gate2016-2-51

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hey Bhagyashree are you sure of the subset diagram you have posted above because i don’t think that every cascade rollback schedule is recoverable.

Another reason i think it is wrong is that according to your diagram, every irrecoverable schedule is recoverable. But you can say that it represents that everything that is recoverable is represented by the inner box and everything that is not recoverable is represented by outer box. Then in that case your innermost part will go wrong i.e. strict and cascade-less relation.

please can you share the source of that diagram.

Answer 1

Answer should be D

Answer 2

If two transactions(T1 and T2) uses the same data,

T1(WRITE) --> T2(READ)       #T2 reads the data written by T1

Cascading roll back ensures recoverability by rolling back T2 when T1 rollback.(III option is true)

cascadeless schedule ensures one transaction(T2) should come after first transaction(T1) commits.

Hence Cascade less schedule is also recoverable and  ensures no roll back of 2nd transaction(T2) as it was postponed.(II option is true)

Strict schedule is more strict which does not allow even,

T1(WRITE) --> T2(WRITE)     #T2 wants to write T1's written data

T1(READ) --> T2(WRITE)       #T2 wants to write T1's read data

then T2 should come after T1 commits.(I option is true)

option iv: wrong as cascading does not happen in strict schedule.

option v: Not neccessary.

 

Answer: D

Comments

Actually the question says “SCHEDULE” and not transactions. Therefore in a schedule, it is possible to have both irrecoverable as well as cascading rollback transactions, BUT if a schedule is cascade less then we are sure that DIRTY READ is not possible, hence recoverable too.

Please argue your counterpoint. Because even this is my personal conclusion.