Semi-join strategies are techniques for query processing in distributed database system. Which of the following is a semi-join technique ?
Only the joining attributes are sent from one site to another and then only the required rows are returned.
In distributed query processing, shipping a whole relation between sites is costly. A semi-join reduces this transfer cost.
Step 1 — Site 1 projects its relation on the joining attribute(s) and sends only this column set to site 2.
Step 2 — Site 2 joins the received values with its relation and keeps only the rows that actually match (the semi-join \(S \ltimes R\)).
Step 3 — Only these required rows are sent back to site 1, where the final join is completed.
So the technique sends only the joining attributes and returns only the required rows. Sending all attributes defeats the purpose of reducing transfer, and returning all of the rows gives no reduction on the way back, so the other descriptions do not describe a semi-join.
Only the joining attributes are sent from one site to another and then only the required rows are returned. option is correct.
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