Managing Multiuser Databases Questions and Answers

Answer: A

Correct answer: True

Transactions are concurrent when their execution overlaps in time, whether or not they touch the same data. The whole apparatus of locking and isolation levels exists to make concurrent transactions behave as though they had run one after another, which is what serializability means.

Answer: C

Correct answer: Dirty read

A dirty read is reading data that another transaction has changed but not yet committed. If that transaction subsequently rolls back, the value read never existed in the database at all, and any decision taken on it is unfounded. Only the read uncommitted isolation level permits it.

Answer: D

Correct answer: Both the first and third answers above are correct.

The two statements are the same responsibility described at two levels. The DBA facilitates the development and use of the database, and that facilitation covers the development, operation and maintenance of the database and its applications. Creating and populating tables is one task within that, not a statement of the overall responsibility.

Answer: D

Correct answer: durable

Durability means that once a transaction has committed, its changes are permanent and survive any subsequent failure. It is achieved by writing to a log on stable storage before the commit is acknowledged.

Answer: A

Correct answer: forward only.

A forward-only cursor can move through the recordset in one direction and cannot return to a row it has passed. It is the cheapest kind, since nothing needs to be retained behind the current position, and it is the right choice whenever the application simply processes each row once.

Answer: B

Correct answer: False

What is described is a nonrepeatable read: rows already seen have been modified or deleted by a committed transaction. A phantom read is the appearance of new rows that were not there before. The distinction decides which isolation level is needed, since repeatable read prevents the first but not the second.

Answer: A

Correct answer: Read committed

Read committed guarantees only that data read has been committed by someone, so dirty reads are impossible. It takes no long-term read locks, so a row may change between two reads within the same transaction - a non-repeatable read - and new rows may appear, a phantom. It is the default level in most systems, as a compromise between correctness and concurrency.

Answer: A

Correct answer: True

This is the definition of a transaction and it is the atomicity property stated in full. The series of actions is treated as a single indivisible unit, so a transfer between two accounts either debits one and credits the other or does neither - the database is never left with the money in neither place.

Answer: A

Correct answer: True

The DBA’s role is to facilitate the development and use of the database rather than to own the data or to write the applications. In practice that covers design support, security administration, backup and recovery, performance, configuration control and the arbitration of conflicting requirements between user groups.

Answer: D

Correct answer: Shared lock

A shared lock permits other transactions to read the item but not to change it, so any number of readers may hold one at once. An exclusive lock permits neither reading nor writing by anyone else. The pair is what allows concurrent reads to proceed freely while any write is serialised.