Antony Asish

01 Mar 2026

SQL Server: Read Committed vs Read Committed Snapshot (RCSI)

Most production systems use Read Committed — but many teams don’t fully understand how it behaves under heavy concurrency.

If your application experiences:

Blocking issues
Slow APIs during peak hours
Read queries waiting for updates

You need to understand this

What is READ COMMITTED?

Read Committed is the default isolation level in SQL Server.

It prevents Dirty Reads But allows Non-Repeatable Reads and Phantom Reads

How it works (Without Snapshot)

When a transaction reads, SQL Server places a Shared Lock (S)
When a transaction writes, SQL Server places an Exclusive Lock (X)
Shared locks block writers
Exclusive locks block readers and writers

Result:

Read and Write operations can block each other.

What is READ COMMITTED SNAPSHOT (RCSI)?

RCSI is an enhanced version of Read Committed that uses Row Versioning instead of Shared Locks for reads.

Instead of waiting, readers access the last committed version of a row stored in tempdb.

Readers don’t block writers. Writers don’t block readers. Still prevents Dirty Reads

How to Enable RCSI

RCSI is enabled at the database level:

ALTER DATABASE YourDatabase

SET READ_COMMITTED_SNAPSHOT ON;

⚠ Requires exclusive database access during change.

After enabling, existing Read Committed queries automatically use row versioning.

No application code change required.

How It Works Internally

Without Snapshot:

Reads → Shared Lock
Writes → Exclusive Lock
Blocking happens

With RCSI:

Reads → No Shared Lock
Writes → Exclusive Lock
Old row versions stored in tempdb
Readers access previous committed data

Behavior Comparison: Read Committed vs RCSI

Dirty Reads

Read Committed → Not Allowed
RCSI → Not Allowed

Read Blocks Write

Read Committed → Yes (Shared lock blocks update)
RCSI → No (Reads use row versioning)

Write Blocks Read

Read Committed → Yes (Exclusive lock blocks read)
RCSI → No (Reader gets last committed version)

Uses tempdb

Read Committed → No
RCSI → Yes (Stores previous row versions)

Code Changes Needed

Read Committed → No
RCSI → No (Works automatically after enabling)

Advantages of RCSI

Reduces blocking dramatically

Improves API performance
Better concurrency
No code modification required
Ideal for high-read systems

Disadvantages of RCSI

Increased tempdb usage
Slight storage overhead for row versions
Doesn’t prevent non-repeatable reads
Doesn’t prevent phantom reads

When Should You Use RCSI?

Use RCSI if:

You run a high-traffic web application
Your read queries frequently get blocked
You want better concurrency without changing code
You’re experiencing blocking in production

Avoid or evaluate carefully if:

tempdb is already heavily loaded
You require full transaction-level consistency (consider SNAPSHOT instead)

Real-World Insight

In modern web APIs (especially .NET + SQL Server stacks), enabling RCSI often:

Reduces blocking incidents
Improves response times
Reduces deadlocks

It’s one of the most underused performance improvements in SQL Server.

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