Managing Multiuser Databases Questions and Answers

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.

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.

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.

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on their own copies of the same record and each writing back a result computed from a value the other has already superseded.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.

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.

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.

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.

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.

Answer: B

Correct answer: False

The organisation-wide function is data administration, which concerns data as a corporate resource across all systems. Database administration is narrower, applying to a particular database and its applications - its design, security, backup, performance and configuration.

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.