When it comes to SQL Server, there are various isolation levels that determine how transactions interact with each other. Two commonly confused isolation levels are Read Committed Snapshot and Snapshot Isolation. In this article, we will explore the differences between these two isolation levels and how they can be used effectively.

Read Committed Snapshot

Read Committed Snapshot is not an isolation level itself, but rather a behavior that can be activated within the Read Committed isolation level. By enabling the Read Committed Snapshot property at the database level, you can avoid blocking between readers (SELECT) and writers (INSERT/UPDATE/DELETE) without changing anything in the application.

Let’s take a look at an example:

ALTER DATABASE SQLAuthority SET READ_COMMITTED_SNAPSHOT ON
ALTER DATABASE SQLAuthority SET ALLOW_SNAPSHOT_ISOLATION ON

In this scenario, if there is a blocking issue between a reader and a writer, enabling the Read Committed Snapshot property allows the reader to access the last committed data without being blocked by the writer. This can greatly improve performance and concurrency in your database.

Snapshot Isolation

Snapshot Isolation is a new isolation level introduced in SQL Server 2005. Unlike Read Committed Snapshot, Snapshot Isolation requires a change in the application as it uses a different isolation level. To enable Snapshot Isolation, you need to modify the database settings and set ALLOW_SNAPSHOT_ISOLATION to ON:

ALTER DATABASE SQLAuthority SET ALLOW_SNAPSHOT_ISOLATION ON

Additionally, you need to change the isolation level of the connection to SNAPSHOT:

SET TRANSACTION ISOLATION LEVEL SNAPSHOT

With Snapshot Isolation, each transaction sees a consistent snapshot of the database as of the start of the transaction. This means that even if another transaction modifies the data, the snapshot transaction will still see the original values until it is committed.

Let’s illustrate this with an example:

USE SQLAuthority
GO

BEGIN TRAN
UPDATE DemoTable SET i = 10 WHERE i = 2

In this scenario, even if the value of i is changed to 10, a transaction using Snapshot Isolation will still see the old record (2, TWO) until the transaction is committed.

It’s important to note that Snapshot Isolation requires careful consideration and testing, as it can lead to increased resource usage and potential conflicts with other transactions. However, when used correctly, it can provide a higher level of concurrency and data consistency.

Conclusion

In summary, SQL Server offers different isolation levels to control how transactions interact with each other. Read Committed Snapshot is a behavior within the Read Committed isolation level that allows readers to access the last committed data without being blocked by writers. Snapshot Isolation, on the other hand, is a new isolation level that provides a consistent snapshot of the database as of the start of the transaction.

Understanding these isolation levels and their implications can help you optimize the performance and concurrency of your SQL Server database. It’s important to carefully evaluate your application’s requirements and test different isolation levels to determine the most suitable option for your specific scenario.

Have you used Snapshot Isolation or Read Committed Snapshot in your SQL Server environment? Share your experiences and scenarios in the comments below!