Two concurrent executing transactions T1 and T2 are allowed to update same stock item say ‘A’ in an uncontrolled manner. In such scenario. Following problems may occur:

(a) Dirty read problem

(b) Lost update problem

(c) Transaction failure

(d) Inconsistent database state

Which of the following option is correct if database system has no concurrency module and allows concurrent execution of above two transactions?

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UGC NET Computer Science (Paper 2) Dec 2019 Paper
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  1. (a), (b) and (c) only
  2. (c) and (d) only
  3. (a) and (b) only
  4. (a), (b) and (d) only

Answer (Detailed Solution Below)

Option 4 : (a), (b) and (d) only
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Detailed Solution

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Problems that can occur during concurrent execution of two transactions in an uncontrolled manner:

a) Lost update problem:  Lost update problem occurs when two transactions that access the same database items have their operations interleaved in a way that makes the value of some database item incorrect.

b) Dirty read problem: It occurs when a transaction is allowed to retrieve or update a record that has been updated by another transaction. Because it has not yet been committed, there is always a possibility that it will never be committed but rather rollback.

d) Inconsistent database state: It occurs when a transaction reads several values, but a second transaction updates some of these values during the execution of first. This causes an inconsistency. For example: if one transaction is calculating an aggregate summary function on a number of records while other transactions are updating some of records.

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