Multiple choices questions in DBMS, Transaction isolation levels in DBMS, Which transaction level permits concurrently executing interleaved transactions? How to handle concurrent transactions under various transaction isolation levels?
DBMS solved MCQ - Transaction isolation level - Find the appropriate isolation level for a concurrent transaction
1. Consider the transactions T1 and T2 that are executing concurrently as given below. Which of the following isolation levels can permit such concurrently executing interleaved transactions to proceed?
|
T1 |
T2 |
|
Begin_transaction
Read(A)
Read(B) Commit; |
Begin_transaction
Read(A) Read(B)
Commit; |
a) Serializable
b) Repeatable read
c) Read committed
d) All of the above
|
Answer: (d) All of the above The given schedule consists of transactions that are read-only. Hence, the above sequence of operations can occur at all levels of isolation since the transactions only read data without writing. If all transactions are read-only transactions in a schedule, then that schedule is a serializable schedule.What is dirty read? Dirty read is the one where a transaction consumes a data item that was updated by another ongoing not-yet-committed transaction. In this question, no WRITE operation is involved.In the question, the schedule does not have any WRITE operations. So, dirty reads are not possible. For this reason, SERIALIZABLE, REPEATABLE READ and READ COMMITTED isolation levels permit the given schedule. |
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