Solution

The correct answer is A transaction reads a data item after it is written by an uncommitted transaction.

Key PointsAn irrecoverable error is an error that does not permit the database to return to a consistent state after a failure. They are primarily caused by inconsistencies that occur during transactions. Looking at the provided scenarios:

1. A transaction writes a data item after it is read by an uncommitted transaction  -
   • This is known as a Dirty Write. This is not necessarily an irrecoverable error, as a rollback operation can undo the changes if the uncommitted transaction fails.
2. A transaction reads a data item after it is read by an uncommitted transaction -
   • This is known as a Dirty Read. This is not necessarily an irrecoverable error. Rollback operations can be implemented to maintain consistency.
3. A transaction reads a data item after it is written by a committed transaction -
   • This is a normal operation within DBMS systems and doesn't lead to an irrecoverable error.
4. A transaction reads a data item after it is written by an uncommitted transaction -
   • This results in a Dirty Read, which can lead to an irrecoverable error under specific circumstances:
     • if the transaction that issued the read operation commits before the transaction that issued the write operation, and then the system fails which causing the uncommitted transaction to abort.
     • In this case, the committed transaction read and used uncommitted data, which can't be rolled back, leading to an irrecoverable state.

So, scenario 4 can cause an irrecoverable error, but it's worth noting that the specific set of circumstances to cause such an error is relatively rare in well-designed systems that use transaction isolation levels to avoid these types of issues.