Difference between RCSI and Snapshot Isolation

 RCSI provides statement-level consistent reads with minimal blocking and no application changes, while Snapshot Isolation provides transaction-level consistency using the same row versioning infrastructure but adds update conflict detection and guarantees a stable point-in-time view throughout the transaction.

Feature	RCSI (Read Committed Snapshot Isolation)	Snapshot Isolation (SI)
Isolation Level	Modified READ COMMITTED	Separate Isolation Level
Introduced In	SQL Server 2005	SQL Server 2005
Uses Row Versioning	Yes	Yes
Uses TempDB Version Store	Yes	Yes
Prevents Dirty Reads	Yes	Yes
Reader Blocks Writer	No (mostly)	No (mostly)
Writer Blocks Reader	No (mostly)	No (mostly)
Shared Locks for Reads	Not required for data reads	Not required for data reads
Consistency Scope	Statement Level	Transaction Level
Snapshot Taken When	At the start of each statement	At the start of transaction
Data Seen by SELECT	Latest committed data when statement starts	Data as of transaction start
Multiple SELECTs in Same Transaction	Can see different committed data	Always see same version
Repeatable Reads Guaranteed?	No	Yes
Phantom Reads Possible?	Yes (between statements)	No
Non-Repeatable Reads Possible?	Yes (between statements)	No
Update Conflict Detection	No	Yes
Error 3960 Possible?	No	Yes
Application Code Changes Required	No	Yes
SET TRANSACTION ISOLATION LEVEL Needed	No	Yes
Enable at Database Level	Yes	Yes
Suitable for Existing Applications	Excellent	Requires code changes
Reporting Consistency	Medium	High
Long Running Transactions	Not ideal for consistent reporting	Excellent
Concurrency	Very High	High
TempDB Usage	High	High
Version Store Usage	High	High
Most Commonly Used	Yes	Less Common
Typical Use Case	OLTP Systems	Financial Reporting / Auditing