Published on

December 26, 2008

Understanding Row Level Versioning in SQL Server

When it comes to managing data in SQL Server, one concept that often comes up is Row Level Versioning. This feature, introduced in SQL Server 2005, allows for improved concurrency and performance when dealing with data modifications. In this article, we will explore what Row Level Versioning is, how it works, and its advantages.

What is Row Level Versioning?

In simple terms, Row Level Versioning is a mechanism that allows SQL Server to keep track of changes made to data at the row level. It works by storing multiple versions of a row in a separate storage area called the version store. Each version is associated with a specific transaction, allowing for the retrieval of previous versions of a row even while new changes are being made.

Traditionally, when a SELECT statement is executed on a table with committed read, it would wait for any ongoing transactions to be committed before returning the result. However, with Row Level Versioning, SELECT statements can execute without waiting, returning the last committed data. This improves concurrency and reduces blocking.

Advantages of Row Level Versioning

There are several advantages to using Row Level Versioning in SQL Server:

  • Improved Concurrency: By allowing SELECT statements to access the last committed value of a row without blocking, Row Level Versioning increases concurrency and reduces the chances of deadlocks.
  • Reduced Locking: SELECT statements do not lock data when reading, reducing the load on the server and decreasing the chances of lock escalation.
  • Performance Optimization: Row Level Versioning improves the performance of triggers, online index creation, and Multiple Active Result Sets (MARS) by utilizing the version store instead of scanning the transaction log.

Implementing Row Level Versioning

To enable Row Level Versioning in SQL Server, you need to set the appropriate isolation levels. There are two isolation levels that support Row Level Versioning:

  • Snapshot Isolation (SI): This isolation level allows SELECT statements to access the last committed value of a row without blocking.
  • Read Committed Snapshot Isolation (RCSI): Similar to SI, RCSI also allows SELECT statements to access the last committed value of a row without blocking, but it provides a higher level of consistency.

Once the isolation levels are set, SQL Server will use the TempDB database to store the additional versions of rows. It is important to manage and maintain the TempDB database properly, as it can have a significant impact on resource usage.

Conclusion

Row Level Versioning is a powerful feature in SQL Server that allows for improved concurrency, reduced locking, and optimized performance. By keeping track of changes made to data at the row level, SQL Server can provide consistent and efficient access to the last committed values of rows. However, it is important to consider the resource usage and potential performance impact when implementing Row Level Versioning.

Overall, Row Level Versioning is a valuable tool for managing data in SQL Server, and understanding its concepts and advantages can greatly enhance your database performance and concurrency.

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