Distributed Databases
A transaction manager is which of the following?

All of these
Maintains a log of transactions
Maintains appropriate concurrency control
Maintains before and after database images

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Distributed Databases
A semijoin is which of the following?

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

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