Tuesday, November 30, 2021

DBMS MCQ - When do we say that a schedule is not conflict serializable

Multiple choices questions in DBMS, When can we mention that a schedule is not a conflict serializable schedule? Why a schedule is said to be non-conflict serializable?

DBMS MCQ - Non-conflict serializable schedule

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1. A schedule is said to be non-conflict serializable if it has one or more of the following.

a) Blind writes

b) Blind reads

c) Conflicting instructions

d) Non-conflicting instructions

Answer: (a) Blind writes and (c) Conflicting instructinos

A schedule is not conflict serializable if it is not conflict equivalent to a serial schedule. And, it is not conflict equivalent if it has two or more transactions that have conflicting instructions (eg. read-write conflict, write-read conflict, and write-write conflict).

Also, the schedules that have transactions consisting of blind writes are also not conflict serializable schedules.

Following schedule is not a conflict serializable schedule because read(A) of T1 conflict with write(A) of T2. Also, write(A) is a blind write (blind write - a write operation executed by a transaction to update a data item without executing a read on the same data item is called blind write).

T1

T2

Read(A)

 

Write(A)

 

Write(A)

 

 

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