GATE | GATE-CS-2006 | Question 69

Consider the relation enrolled(student, course) in which (student, course) is the primary key, and the relation paid(student, amount), where student is the primary key. Assume no null values and no foreign keys or integrity constraints. Assume that amounts 6000, 7000, 8000, 9000 and 10000 were each paid by 20% of the students. Consider these query plans (Plan 1 on left, Plan 2 on right) to “list all courses taken by students who have paid more than x”.

A disk seek takes 4ms, disk data transfer bandwidth is 300 MB/s and checking a tuple to see if amount is greater than x takes 10 micro-seconds. Which of the following statements is correct?
(A) Plan 1 and Plan 2 will not output identical row sets for all databases.
(B) A course may be listed more than once in the output of Plan 1 for some databases
(C) For x = 5000, Plan 1 executes faster than Plan 2 for all databases.
(D) For x = 9000, Plan I executes slower than Plan 2 for all databases.

Answer: (C)
Explanation: See question 3 of https://www.geeksforgeeks.org/database-management-systems-set-9/amp/
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