The semijoin approach saves network traffic.
Answer: A
Correct answer: True
The semijoin sends only the joining column values out and receives only the matching rows back, so both directions carry far less than a full transfer of either table. Where the network is the bottleneck - which in a distributed database it usually is - that saving governs the cost of the query.
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In a semijoin, only the joining attribute is sent from one site to another, and then only the required rows are returned.
Answer: A
Correct answer: True
This is the definition. The joining attribute alone is sent to the other site, which matches it against its own rows and returns only those that participate in the join. The saving comes from restricting both what is sent and what comes back, and it is what makes distributed joins practicable over a slow link.
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The problem of concurrency control is more complex in a distributed database.
Answer: A
Correct answer: True
A centralised system controls concurrency with one lock manager holding the whole picture. Distributing it means locks held at several sites, messages that may be delayed or lost, and deadlocks whose cycle spans sites and which no single site can see. This is why timestamping, which avoids locks altogether, is attractive in a distributed setting.
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