Managing Multiuser Databases Questions and Answers

Answer: B

Correct answer: False

Locks placed by command are explicit; implicit locks are those the DBMS places on its own initiative to enforce the isolation level in force. The two terms classify locks by who asked for them, which is a different question from whether the lock is shared or exclusive.

Answer: B

Correct answer: False

The term is lock granularity, not lock sizing. It describes whether the lock covers a row, a page, a table or the whole database, and the choice trades concurrency against the overhead of tracking a large number of locks.

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.

Answer: B

Correct answer: explicit locks

An explicit lock is one the application asks for by issuing a command; an implicit lock is one the DBMS places on its own initiative to enforce isolation. The distinction is about who requests the lock. Shared and exclusive describe something different - what the lock permits others to do - so the two classifications are independent.

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.

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.

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.

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.

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.

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.