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

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:

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