GATE2003-29, 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

APPROPIATE ANSWER IS D

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1. A transaction writes a data item after it is read by an uncommitted transaction 

 Blind Write. So commit or not, does not matter.

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Thanks man :-)

You only made the point clear

Answer 1

D) This is dirty read. In case if transaction reading uncommited data commits, irrecoverable error occurs of uncommited transaction fails. So D is answer

B) This is non issue.Both transaction reading data.

C) This is non issue.

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.

Answer 2

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

Answer 3

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 4

APPROPIATE ANSWER IS D

Comments

cn u explain this by easy example?

Answer 5

ANS ---     Option D