Managing Multiuser Databases Questions and Answers

Q: Which of the following allows dirty reads, nonrepeatable reads and phantom reads to occur?

Correct answer: Read uncommittedRead uncommitted is the weakest level and prevents nothing: dirty reads, nonrepeatable reads and phantoms are all possible, because the transaction takes no read locks and respects none. It is used only where speed matters more than accuracy - an approximate count over a large table, for instance.

Q: Which of the following locks the item from access of any type?

Correct answer: Exclusive lockAn exclusive lock excludes everyone else entirely: no other transaction may read the item or change it while the lock is held. It is what a writing transaction needs. A shared lock is the weaker form, permitting concurrent readers but no writer, which is why several may be held on the same item at once.

Q: The size of a lock is called:

Correct answer: lock granularity.Granularity is the size of the thing locked - a row, a page, a table, or the whole database. It is a trade: a coarse lock is cheap to manage but blocks many transactions, while a fine one allows far more concurrency at the cost of a great many locks to track. The DBMS may escalate from fine to coarse when the count grows.

Q: A cursor type in which the application sees the data as they were at the time the cursor was opened is called:

Correct answer: static.A static cursor takes a snapshot of the data when it is opened and shows that snapshot throughout, so changes made by others afterwards are invisible. It is the opposite of the dynamic cursor, and the keyset cursor sits between them, showing changes to the rows originally selected but not rows added later.

Q: Locks placed by the DBMS are called ________ .

Correct answer: implicit locksLocks placed by the DBMS on its own initiative, to enforce the isolation level in force, are implicit. An explicit lock is one the application requests by command. Most locking in practice is implicit, which is the point of declaring an isolation level: the developer states what must be prevented and leaves the DBMS to arrange it.

Q: Resource locking is a process to prevent multiple applications from obtaining copies of the same record when the record is about to be changed.

Correct answer: TrueResource locking is exactly this: preventing two applications from working on their own copies of the same record and each writing back a result computed from a value the other has already superseded. Without it the second write silently overwrites the first, which is the lost update problem.

Q: The overall responsibility of the DBA is to facilitate the development and use of the database.

Correct answer: TrueThe 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.

Q: The size of the lock is referred to as the lock granularity.

Correct answer: TrueLock granularity is the size of the item locked, from a single row up to the whole database. Fine granularity allows more transactions to proceed at once but requires far more locks to be held and checked; coarse granularity is cheap to administer but blocks transactions that would not really have conflicted.

Q: A durable transaction is one in which all committed changes are permanent.

Correct answer: TrueDurability is the last of the four ACID properties: once a transaction has committed, its changes survive whatever happens afterwards, including a crash. It is achieved by writing the change to the transaction log before acknowledging the commit, so that the change can be reapplied during recovery even if it never reached the datafiles.

41.

Which of the following allows dirty reads, nonrepeatable reads and phantom reads to occur?

A.Read committed
B.Read uncommitted
C.Repeatable read
D.Serializable

Answer: B

Correct answer: Read uncommitted

Read uncommitted is the weakest level and prevents nothing: dirty reads, nonrepeatable reads and phantoms are all possible, because the transaction takes no read locks and respects none. It is used only where speed matters more than accuracy - an approximate count over a large table, for instance.

Discuss in Forum

42.

Which of the following locks the item from access of any type?

A.Implicit lock
B.Explicit lock
C.Exclusive lock
D.Shared lock

Answer: C

Correct answer: Exclusive lock

An exclusive lock excludes everyone else entirely: no other transaction may read the item or change it while the lock is held. It is what a writing transaction needs. A shared lock is the weaker form, permitting concurrent readers but no writer, which is why several may be held on the same item at once.

Discuss in Forum

43.

The size of a lock is called:

A.implicit lock.
B.lock granularity.
C.exclusive lock.
D.shared lock.

Answer: B

Correct answer: lock granularity.

Granularity is the size of the thing locked - a row, a page, a table, or the whole database. It is a trade: a coarse lock is cheap to manage but blocks many transactions, while a fine one allows far more concurrency at the cost of a great many locks to track. The DBMS may escalate from fine to coarse when the count grows.

Discuss in Forum

44.

A cursor type in which the application sees the data as they were at the time the cursor was opened is called:

A.forward only.
B.static.
C.keyset.
D.dynamic.

Answer: B

Correct answer: static.

A static cursor takes a snapshot of the data when it is opened and shows that snapshot throughout, so changes made by others afterwards are invisible. It is the opposite of the dynamic cursor, and the keyset cursor sits between them, showing changes to the rows originally selected but not rows added later.

Discuss in Forum

45.

What is the overall responsibility of the DBA?

A.Facilitate the development and use of the database
B.Create and populate tables
C.Development, operation, and maintenance of the database and its applications
D.Both the first and third answers above are correct.

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.

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46.

Locks placed by the DBMS are called ________ .

A.implicit locks
B.explicit locks
C.exclusive locks
D.shared locks

Answer: A

Correct answer: implicit locks

Locks placed by the DBMS on its own initiative, to enforce the isolation level in force, are implicit. An explicit lock is one the application requests by command. Most locking in practice is implicit, which is the point of declaring an isolation level: the developer states what must be prevented and leaves the DBMS to arrange it.

Discuss in Forum

47.

Resource locking is a process to prevent multiple applications from obtaining copies of the same record when the record is about to be changed.

A.True
B.False

Answer: A

Correct answer: True

Resource locking is exactly this: preventing two applications from working on their own copies of the same record and each writing back a result computed from a value the other has already superseded. Without it the second write silently overwrites the first, which is the lost update problem.

Discuss in Forum

48.

The overall responsibility of the DBA is to facilitate the development and use of the database.

A.True
B.False

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.

Discuss in Forum

49.

The size of the lock is referred to as the lock granularity.

A.True
B.False

Answer: A

Correct answer: True

Lock granularity is the size of the item locked, from a single row up to the whole database. Fine granularity allows more transactions to proceed at once but requires far more locks to be held and checked; coarse granularity is cheap to administer but blocks transactions that would not really have conflicted.

Discuss in Forum

50.

A durable transaction is one in which all committed changes are permanent.

A.True
B.False

Answer: A

Correct answer: True

Durability is the last of the four ACID properties: once a transaction has committed, its changes survive whatever happens afterwards, including a crash. It is achieved by writing the change to the transaction log before acknowledging the commit, so that the change can be reapplied during recovery even if it never reached the datafiles.

Discuss in Forum

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