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Wednesday, 8 December 2021

DBMS MCQ - What is dirty read in DBMS transactions

Multiple choices questions in DBMS, What is dirty read in database? What will go wrong if a transaction is allowed to read a data item that was written by an uncommitted transaction? Why dirty read is not a preferable one? Can we permit a transaction that has used a data item written by another uncommitted transaction to commit? What are the problems with dirty read?

DBMS MCQ - Dirty read by transactions

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1. If a transaction is allowed to read a data item that was produced by an uncommitted transaction, what do we call this read? 

a) Repeatable read

b) Phantom read

c) Dirty read

d) Conflict read

Answer: (c) Dirty read

Reading the value of a data item that was produced by an uncommitted transaction is referred as dirty read problem. This will be avoided if we permit transactions to read the values that are produced by committed transactions.

Possible problems with dirty read in database

  • It may result in non-recoverable schedule. This is possible if a transaction T1 consumes the data written by T2 but commits before T2, and T2 decided to rollback.
  • It may lead to cascading rollback. This situation is possible if a transaction T3 consumes the data written by T2 which in turn read the data from another transaction T1, and T1 decided to rollback.
  • A dirt read can cause duplicate rows to be returned where none exist. alternatively, a dirty read can cause no rows to be returned when one (or more) exists. In some cases, dirty reads can return data that was never in the database at all (eg. rolled back before commit).

How you may avoid dirty read?

You can set the TRANSACTION ISOLATION LEVEL as READ COMMITTED. This specifies that the statements cannot read data that has been modified but not committed by other transaction. This prevents dirty reads.


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