Question

Which of the following is a semi join?

• A. Only the joining attributes are sent from one site to another and then all of the rows are returned
• B. All of the attributes are sent from one site to another and then only the required rows are returned
• C. Only the joining attributes are sent from one site to another and then only the required rows are returned
• D. All of the attributes are sent from one site to another and then only the required rows are returned

Hint:

A semi join reduces data transfer costs in distributed databases by sending only the joining attributes, improving efficiency.

Answer 1

The Correct Option is C

Step 1: Understanding Semi Join
A semi join is a join operation that transfers only the joining attributes between two relations rather than all attributes. This reduces the communication cost in distributed databases.
Step 2: Evaluating the Options
- (A) Incorrect: In a semi join, only the required rows are returned, not all rows.
- (B) Incorrect: Sending all attributes increases overhead, violating the goal of a semi join.
- (C) Correct: The semi join only transfers the joining attributes and then retrieves only the required rows.
- (D) Incorrect: This describes a regular join rather than a semi join.