GATE CSE 2003 | Question: 29, ISRO2009-73

Question

Which of the following scenarios may lead to an irrecoverable error in a database system?

• A. A transaction writes a data item after it is read by an uncommitted transaction
• B. A transaction reads a data item after it is read by an uncommitted transaction
• C. A transaction reads a data item after it is written by a committed transaction
• D. A transaction reads a data item after it is written by an uncommitted transaction

Comments

Thanks man :-)

You only made the point clear

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Option D is Correct

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$$\underline{\text{Analogy}}$$

Imagine you and your friend are writing an exam, and your friend cheats by copying your answer and submits the answer sheet before you submit yours; later you realize that you made some mistakes.

You can make the appropriate changes, but your friend can't. The damage your friend has incurred is irrecoverable.

Answer 1

• A. Here if transaction writing data commits , then transaction which read the data might get phantom tuple/ Unrepeatable error. Though there is no irrecoverable error possible even in this option.
• B. This is non issue. Both transaction reading data.
• C. This is non issue.
• D. This is dirty read. In case if transaction reading uncommitted data commits, irrecoverable error occurs of uncommitted transaction fails. So (D) is answer

Comments

Can I please know as to why recovery from an unrepeatable read is possible in this scenario but a recovery from a dirty read is not?

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@aiyyar.aarushi I think the reason is that since unrepeatable error is against the isolation property, database will undo the changes that was introduced , using the logs .In case of option (d) it is not that every dirty read leads to unrecoverable schedule, but it "may lead to an irrecoverable error" as the question has asked about "may lead to'' therefore option d is the right one since "if transaction reading uncommitted data commits, before the other transaction that did the write commits,  then irrecoverable error occurs if uncommitted transaction fails".

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@aiyyar.aarushi in option D, if you abort the transaction which performed dirty read (after the other transaction has committed) no issue. If you abort the transaction which performed the write (after the other transaction has committed), you must rollback both transactions to ensure atomicity but if you abort the committed transaction, you would violate durability.

Answer 2

 

I hope now it is clear which is the only case where IRRECOVERABILITY ARISES.

That is only when a particular transaction reads from a uncommited Transacation and , commits before the commit of the transaction whose data it was reading.

Comments

how this is phantom read in second image?

Answer 3

D. If Data is uncommitted , then transactions reads a data after write always give error.

Answer 4

D

Option C is a normal operation. Option B is also fine as no write operation is involved. Option A can be recovered, but option D can't be.

Comments

after write by uncommited transaction again write is a problem.

but after write by uncommited transaction then read why it will be problem?

Answer 5

A transaction is reading a data item after it is written by an uncommitted transaction, the schedule will be irrecoverable.

This is irrecoverable Schedule because T2 reads a updated value before the commit operation of T1.

Answer 6

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