SQL SERVER – The Comprehensive Guide to STATISTICS_NORECOMPUTE

SQL Server, a robust database engine, is designed with numerous options and settings to optimize query performance and tailor its behavior to your needs. One such feature that plays a pivotal role in managing your SQL Server databases’ performance is the STATISTICS_NORECOMPUTE option.

Here is the sample script:

-- Create an index with STATISTICS_NORECOMPUTE = ON
CREATE INDEX IX_YourTable_YourColumn 
ON YourTable(YourColumn) 
WITH (STATISTICS_NORECOMPUTE = ON);
GO

Understanding STATISTICS_NORECOMPUTE

In the context of SQL Server, statistics are a fundamental component of the Query Optimizer. They provide a histogram of the data distribution in an indexed column, which the Query Optimizer uses to devise efficient query plans. By default, SQL Server automatically updates these statistics whenever a significant amount of data in the indexed column is added, changed, or removed.

The STATISTICS_NORECOMPUTE option, used within CREATE INDEX or ALTER INDEX statements, can override this default behavior. When STATISTICS_NORECOMPUTE is set to ON, SQL Server is instructed not to automatically update the statistics for that index. Conversely, when STATISTICS_NORECOMPUTE is set to OFF (or not mentioned at all), SQL Server continues to update the statistics automatically.

When to Use STATISTICS_NORECOMPUTE

The decision to use STATISTICS_NORECOMPUTE should not be taken lightly. Here are some key considerations:

Understanding Your Data and Workload

If your data and workload are stable with minimal changes to data distribution, then setting STATISTICS_NORECOMPUTE to ON may be beneficial. Frequent statistics updates in a stable environment may be unnecessary and could add extra overhead.

Regular Manual Updates

When STATISTICS_NORECOMPUTE is set to ON, it’s crucial to have a process in place that manually updates statistics regularly. You could schedule a SQL Server Agent job to do this during off-peak hours to minimize disruptions.

Performance Implications

Performance may degrade over time if statistics are not updated regularly (either manually or automatically). Outdated statistics can lead to inefficient query execution plans.

Database Maintenance

Consider how STATISTICS_NORECOMPUTE fits into your overall database maintenance strategy. For example, index rebuilding also updates statistics, which could render regular statistics updates redundant if you rebuild indexes frequently.

Thorough Testing

Always test the effects of STATISTICS_NORECOMPUTE in a non-production environment before implementing it in your production setup. Monitor the performance impacts and adjust your strategy accordingly.

The Middle Ground

Consider using the AUTO_UPDATE_STATISTICS_ASYNC option. This allows SQL Server to update statistics asynchronously, eliminating potential blocking of queries while statistics are updated.

ALTER DATABASE [YourDatabase]
SET AUTO_UPDATE_STATISTICS_ASYNC ON;
GO

In Conclusion

While it’s generally true that updated statistics lead to efficient query plans, there are scenarios where automatic updates might not be desirable. The process of updating statistics can be resource-intensive. If you have a very large table and changes are frequent, automatic updating of statistics might cause performance issues.

However, remember that SQL Server is a powerful tool, and with great power comes great responsibility. Understanding the inner workings of SQL Server, including the role of statistics and the STATISTICS_NORECOMPUTE option, is crucial to effectively managing SQL Server performance. While the STATISTICS_NORECOMPUTE option can be beneficial, its use should be considered carefully based on the specific requirements of your database and workload patterns.

You can always reach out to me on Twitter.

Reference: Pinal Dave (https://blog.sqlauthority.com)