Does the semijoin strategy reduce network traffic in distributed join processing?

Difficulty: Easy

Correct Answer: Applies — semijoins are used specifically to cut data shipping

Explanation:


Introduction / Context:
Network bandwidth is a scarce resource in distributed databases. The semijoin technique is a classic optimization to reduce transferred data during joins.


Given Data / Assumptions:

  • Relations to be joined reside on different sites.
  • Join selectivity allows filtering large fractions of non-matching rows.
  • Join keys can be exchanged at low cost compared to full rows.


Concept / Approach:
A semijoin sends projected join keys first, enabling the remote site to eliminate non-matching rows, and then returns only necessary tuples, reducing bytes on the wire.


Step-by-Step Solution:
Project join attribute(s) at the initiating site.Send keys to the remote site.Filter and return only matching tuples.Complete the final join locally with reduced data volume.


Verification / Alternative check:
Cost-based optimizers consider semijoins when the key set is much smaller than the base relation sizes.


Why Other Options Are Wrong:
Semijoins do not require replication, outer joins, or specific schemas; they address transmission cost broadly.


Common Pitfalls:
Using semijoins when the key set is large can negate benefits due to extra passes.


Final Answer:
Applies — semijoins are used specifically to cut data shipping

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