Default transaction isolation in SQL Server:\nAssess the statement:\n\n"READ COMMITTED is the default isolation level."

Difficulty: Easy

Correct Answer: Correct

Explanation:


Introduction / Context:
Transaction isolation level governs how concurrent transactions interact. In SQL Server, the default isolation level for new sessions is READ COMMITTED unless explicitly changed at the session/database level. Understanding this baseline is crucial for diagnosing blocking, deadlocks, and consistency anomalies.


Given Data / Assumptions:

  • No SET TRANSACTION ISOLATION LEVEL overrides have been issued for the session.
  • Database options like READ_COMMITTED_SNAPSHOT may be ON or OFF; this changes implementation, not the named level.
  • We consider standard SQL Server behavior.


Concept / Approach:
Under READ COMMITTED, statements cannot read data modified by other transactions that have not yet committed. With READ_COMMITTED_SNAPSHOT ON, SQL Server implements this using row versioning (nonblocking reads); otherwise it uses shared locks on read. The default level name remains READ COMMITTED in both cases.


Step-by-Step Solution:

Open a new session; query sys.dm_exec_sessions to confirm isolation_level.Observe behavior: dirty reads are prevented; nonrepeatable/phantom reads can occur.Enable READ_COMMITTED_SNAPSHOT and repeat: semantics stay READ COMMITTED, implementation switches to versioning.


Verification / Alternative check:
Use DBCC USEROPTIONS to display the current session’s isolation level. Review database properties for snapshot-based implementation.


Why Other Options Are Wrong:

  • Incorrect: Conflicts with default server behavior.
  • Recovery model / tempdb: Recovery model and database choice do not redefine the default isolation level.
  • RCSI-only: RCSI is an implementation detail for READ COMMITTED, not a different default level.


Common Pitfalls:
Assuming SNAPSHOT is the default; forgetting to measure blocking differences when switching RCSI on.


Final Answer:
Correct

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