Managing Multiuser Databases Questions and Answers

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: Money is defined in Oracle with the Currency data type.

Correct answer: FalseOracle has no Currency datatype. Money is held in a NUMBER column with an explicit precision and scale, such as NUMBER(9,2), which stores the value exactly rather than in binary floating point - and exactness is what matters for money. The Currency type belongs to other products.

Q: An ACID transaction is one that is atomic, consistent isolated, and durable.

Correct answer: TrueThe four properties are atomic, consistent, isolated and durable. Atomic means all or nothing; consistent means the database moves from one valid state to another; isolated means concurrent transactions do not see each other’s intermediate states; and durable means committed changes survive failure.

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.

31.

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

32.

Money is defined in Oracle with the Currency data type.

A.True
B.False

Answer: B

Correct answer: False

Oracle has no Currency datatype. Money is held in a NUMBER column with an explicit precision and scale, such as NUMBER(9,2), which stores the value exactly rather than in binary floating point - and exactness is what matters for money. The Currency type belongs to other products.

Discuss in Forum

33.

An ACID transaction is one that is atomic, consistent isolated, and durable.

A.True
B.False

Answer: A

Correct answer: True

The four properties are atomic, consistent, isolated and durable. Atomic means all or nothing; consistent means the database moves from one valid state to another; isolated means concurrent transactions do not see each other’s intermediate states; and durable means committed changes survive failure.

Discuss in Forum

34.

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

35.

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

36.

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

37.

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

38.

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

39.

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

40.

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

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