Managing Multiuser Databases Questions and Answers

Q: A point of synchronization between the database and the transaction log is generally referred to as a stop point.

Correct answer: FalseA point of synchronisation between the database and its log is a checkpoint. At a checkpoint the buffers are written out and the log records the fact, so recovery need only go back to the last checkpoint rather than to the beginning of the log - which is what keeps recovery time bounded.

Q: Which of the following disallows both dirty reads and nonrepeatable reads, but allows phantom reads?

Correct answer: Repeatable readRepeatable read holds read locks to the end of the transaction, so a row once read cannot be changed by another transaction and dirty and nonrepeatable reads are both prevented. It does not lock rows that do not yet exist, so another transaction may still insert one and a phantom read remains possible.

Q: Locks placed by the DBMS are explicit locks.

Correct answer: FalseLocks placed by the DBMS itself are implicit; explicit locks are those the application requests by command. Getting the pair the right way round matters, because most locking in a well-written application is implicit - the developer declares an isolation level and the DBMS places whatever locks are needed to honour it.

Q: The goal of database security is to ensure that only authorized users can perform authorized activities at authorized times.

Correct answer: TrueAll three qualifications are needed. Authorised users, so that identity is established; authorised activities, so that a user may read what they may not change; and authorised times, since access appropriate during a shift may not be appropriate outside it. Authentication establishes the first and authorisation the other two.

Q: Which of the following refers to a cursor type where changes of any type and from any source are visible?

Correct answer: DynamicA dynamic cursor shows everything: changes, insertions and deletions from any source, made at any time. It is the most current view and correspondingly the most expensive, since the recordset must be re-evaluated as it is traversed. A static cursor is the opposite extreme, showing a snapshot fixed when the cursor was opened.

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: An exclusive lock locks the item from change but not from read.

Correct answer: FalseThat describes a shared lock, which permits others to read but not to change. An exclusive lock excludes access of every kind, reading included, which is what a writing transaction requires. Several shared locks may be held on one item at once; an exclusive lock may not coexist with any other.

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: 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: In optimistic locking, the assumption is made that conflict will occur.

Correct answer: FalseOptimistic locking assumes conflict will not occur: the transaction proceeds without locks and checks at the end whether anything it relied on has changed, rolling back and retrying if so. Pessimistic locking makes the opposite assumption and takes locks before the work begins.

21.

A point of synchronization between the database and the transaction log is generally referred to as a stop point.

A.True
B.False

Answer: B

Correct answer: False

A point of synchronisation between the database and its log is a checkpoint. At a checkpoint the buffers are written out and the log records the fact, so recovery need only go back to the last checkpoint rather than to the beginning of the log - which is what keeps recovery time bounded.

Discuss in Forum

22.

Which of the following disallows both dirty reads and nonrepeatable reads, but allows phantom reads?

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

Answer: C

Correct answer: Repeatable read

Repeatable read holds read locks to the end of the transaction, so a row once read cannot be changed by another transaction and dirty and nonrepeatable reads are both prevented. It does not lock rows that do not yet exist, so another transaction may still insert one and a phantom read remains possible.

Discuss in Forum

23.

Locks placed by the DBMS are explicit locks.

A.True
B.False

Answer: B

Correct answer: False

Locks placed by the DBMS itself are implicit; explicit locks are those the application requests by command. Getting the pair the right way round matters, because most locking in a well-written application is implicit - the developer declares an isolation level and the DBMS places whatever locks are needed to honour it.

Discuss in Forum

24.

The goal of database security is to ensure that only authorized users can perform authorized activities at authorized times.

A.True
B.False

Answer: A

Correct answer: True

All three qualifications are needed. Authorised users, so that identity is established; authorised activities, so that a user may read what they may not change; and authorised times, since access appropriate during a shift may not be appropriate outside it. Authentication establishes the first and authorisation the other two.

Discuss in Forum

25.

Which of the following refers to a cursor type where changes of any type and from any source are visible?

A.Forward only
B.Static
C.Keyset
D.Dynamic

Answer: D

Correct answer: Dynamic

A dynamic cursor shows everything: changes, insertions and deletions from any source, made at any time. It is the most current view and correspondingly the most expensive, since the recordset must be re-evaluated as it is traversed. A static cursor is the opposite extreme, showing a snapshot fixed when the cursor was opened.

Discuss in Forum

26.

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

27.

An exclusive lock locks the item from change but not from read.

A.True
B.False

Answer: B

Correct answer: False

That describes a shared lock, which permits others to read but not to change. An exclusive lock excludes access of every kind, reading included, which is what a writing transaction requires. Several shared locks may be held on one item at once; an exclusive lock may not coexist with any other.

Discuss in Forum

28.

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

29.

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

30.

In optimistic locking, the assumption is made that conflict will occur.

A.True
B.False

Answer: B

Correct answer: False

Optimistic locking assumes conflict will not occur: the transaction proceeds without locks and checks at the end whether anything it relied on has changed, rolling back and retrying if so. Pessimistic locking makes the opposite assumption and takes locks before the work begins.

Discuss in Forum

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