Functional-Dependency
August 29, 2024Database-Management-System
August 29, 2024Database-Management-System
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Question 457
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Two concurrent executing transactions T1 and T2 are allowed to update same stock item say ‘A’ in an uncontrolled manner. In such a 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 allow concurrent execution of above two transactions?
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(a), (b) and (c) only
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(c) and (d) only
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(a) and (b) only
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(a), (b) and (d) only
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Question 457 Explanation:
Following problems can occur when concurrent transactions execute in an uncontrolled manner:
The Lost Update Problem. This 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 items incorrect.
The Temporary Update (or Dirty Read) Problem. This problem occurs when one transaction updates a database item and then the transaction fails for some reason
The Incorrect Summary Problem. If one transaction is calculating an aggregate summary function on a number of database items while other transactions are updating some of these items, the aggregate function may calculate some values before they are updated and others after they are updated.
The Unrepeatable Read Problem. Another problem that may occur is called unrepeatable read, where a transaction T reads the same item twice and the item is changed by another transaction T between the two reads. Hence, T receives
different values for its two reads of the same item.
The Lost Update Problem. This 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 items incorrect.
The Temporary Update (or Dirty Read) Problem. This problem occurs when one transaction updates a database item and then the transaction fails for some reason
The Incorrect Summary Problem. If one transaction is calculating an aggregate summary function on a number of database items while other transactions are updating some of these items, the aggregate function may calculate some values before they are updated and others after they are updated.
The Unrepeatable Read Problem. Another problem that may occur is called unrepeatable read, where a transaction T reads the same item twice and the item is changed by another transaction T between the two reads. Hence, T receives
different values for its two reads of the same item.
Correct Answer: D
Question 457 Explanation:
Following problems can occur when concurrent transactions execute in an uncontrolled manner:
The Lost Update Problem. This 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 items incorrect.
The Temporary Update (or Dirty Read) Problem. This problem occurs when one transaction updates a database item and then the transaction fails for some reason
The Incorrect Summary Problem. If one transaction is calculating an aggregate summary function on a number of database items while other transactions are updating some of these items, the aggregate function may calculate some values before they are updated and others after they are updated.
The Unrepeatable Read Problem. Another problem that may occur is called unrepeatable read, where a transaction T reads the same item twice and the item is changed by another transaction T between the two reads. Hence, T receives
different values for its two reads of the same item.
The Lost Update Problem. This 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 items incorrect.
The Temporary Update (or Dirty Read) Problem. This problem occurs when one transaction updates a database item and then the transaction fails for some reason
The Incorrect Summary Problem. If one transaction is calculating an aggregate summary function on a number of database items while other transactions are updating some of these items, the aggregate function may calculate some values before they are updated and others after they are updated.
The Unrepeatable Read Problem. Another problem that may occur is called unrepeatable read, where a transaction T reads the same item twice and the item is changed by another transaction T between the two reads. Hence, T receives
different values for its two reads of the same item.