This set of RDBMS Multiple Choice Questions & Answers (MCQs) focuses on “Multiversion Schemes, Snapshot Isolation”.
1. In _______ schemes, each write operation creates a new version of Q
a) Multiversion
b) Snapshot isolation
c) Lock based
d) Timestamp
View Answer
Explanation: In multiversion schemes, each write operation creates a new version of Q. When a transaction issues a read operation, the concurrency control manager selects one of the versions of Q to be read.
2. If the first update is overwritten by a second, it is called as a _______ update
a) Useful
b) Overlapping
c) Lost
d) Concurrent
View Answer
Explanation: If the first update is overwritten by a second, it is called as a lost update. This generally occurs whenever two transactions are allowed to write on the database.
3. State true or false: Snapshot isolation prevents lost updates
a) True
b) False
View Answer
Explanation: Snapshot isolation prevents lost updates because it does not allow two transactions to simultaneously write on the same database.
4. Which of the following is a variant of snapshot isolation
a) First committer wins
b) First updater wins
c) More than one of the mentioned
d) None of the mentioned
View Answer
Explanation: First committer wins and first updater wins are two different variations of snapshot isolation.
5. Under ________ the system uses locking mechanism that applies only to updates
a) First updater wins
b) First committer wins
c) First writer wins
d) None of the mentioned
View Answer
Explanation: Under first updater wins, the system uses a locking mechanism that applies only to updates. Read operations do not get affected by this mechanism.
6. The situation in which each pair of transactions has read a data written by the other, but there is no data written by the transactions is called as _______
a) Deadlock
b) Read skew
c) Deadlock skew
d) Write skew
View Answer
Explanation: The situation in which each pair of transactions has read a data written by the other, but there is no data written by the transactions is called as write skew.
7. Oracle uses __________ for the serializable isolation level
a) Multiversion scheme
b) Timestamp protocol
c) Lock based protocol
d) Snapshot isolation
View Answer
Explanation: Oracle uses snapshot isolation for the serializable isolation level. PostgreSQL also uses only snapshot isolation for the serializable isolation level.
8. State true or false: Snapshot isolation has low overhead
a) True
b) False
View Answer
Explanation: Snapshot isolation is attractive in practice because it has a relatively very low overhead.
9. In ________ no two aborts occur unless two concurrent transactions update the same data item.
a) Multiversion scheme
b) Timestamp protocol
c) Lock based protocol
d) Snapshot isolation
View Answer
Explanation: In snapshot isolation no two aborts occur unless two concurrent transactions update the same data item. This makes snapshot isolation attractive but it does not ensure serializability.
10. Which of the following transactions can multiversion two phase locking protocol not differentiate.
a) Read only transactions
b) Update transactions
c) All of the mentioned
d) Double operator transactions
View Answer
Explanation: The multiversion two phase locking protocol cannot differentiate double operator transactions but it can differentiate between read only transactions and update transactions.
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