Multiple choices questions in DBMS, Interview questions in RDBMS, What is phantom read? Example transactions that result in phantom read, define phantom read, given a schedule find the problem in it

RDBMS solved MCQ - Phantom read and concurrency control

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1. What is the problem with the following schedule?

a) Repeatable read problem

b) Lost update problem

c) Phantom read problem

d) Dirty read problem

Answer: (c) Phantom read problem

Transaction T1 finds the number of records (count) of Employee table, updates the salary of all employees using the count, and then commits. Transaction T2 inserts a new record into Employee table and commits.

Suppose that Employee has 50 records before T2 starts. That makes the value of X 50. Now, T2 inserts a new record and commits. Hence, the actual count becomes 51. So, UPDATE statement in T1 updates all the salaries (including the one which is newly inserted – the phantom read) by using the X value as 50 (not as 51).

What is phantom read?

The so-called phantom problem occurs within a transaction when the same query produces different sets of rows at different times. For example, if a SELECT is executed twice, but returns a row the second time that was not returned the first time, the row is a “phantom” row.