OPTIMIZE_FOR_SEQUENTIAL_KEY is an index option introduced in SQL Server 2019 (15.x) aimed at improving the performance of high-concurrency insert scenarios, particularly when dealing with tables that have a high volume of inserts and use a sequential key, such as an IDENTITY column or a SEQUENCE, as their primary key or clustered index key. This option is especially beneficial in mitigating the performance impact of last-page insert contention, a common bottleneck in OLTP (Online Transaction Processing) workloads.

Background

In SQL Server, when multiple transactions concurrently insert rows into a table with a sequential key, they tend to contend for the same "hot" page in the database—the last page of the index where the next sequential value would be inserted. This contention is known as last-page contention or "hotspot" contention and can significantly hinder throughput and scalability in high-concurrency environments.

How OPTIMIZE_FOR_SEQUENTIAL_KEY Works

The OPTIMIZE_FOR_SEQUENTIAL_KEY index option addresses this contention issue by employing a more sophisticated mechanism to manage the allocation of pages and rows for inserts. When this option is enabled on an index with a sequential key, SQL Server dynamically manages the concurrency model for that index, allowing for more efficient use of available system resources and reducing contention on the last page of the index.

Usage

To enable OPTIMIZE_FOR_SEQUENTIAL_KEY, you can specify it in the CREATE INDEX or ALTER INDEX statement. Here's how you can apply it to a new index:

CREATE INDEX [YourIndexName] ON [YourTableName]
(
    [YourSequentialKeyColumn] ASC
)
WITH (OPTIMIZE_FOR_SEQUENTIAL_KEY = ON);

Or, to modify an existing index:

ALTER INDEX [YourIndexName] ON [YourTableName]
SET (OPTIMIZE_FOR_SEQUENTIAL_KEY = ON);

Considerations

• Specific Use Case: It's most beneficial for tables with a high volume of concurrent inserts and a sequential key. It might not be necessary for tables with low insert concurrency or non-sequential keys.

• SQL Server Version: This feature is available starting from SQL Server 2019. Ensure your environment is running an appropriate version of SQL Server that supports this option.

• Monitoring and Testing: As with any performance optimization, it's crucial to monitor the impact of enabling OPTIMIZE_FOR_SEQUENTIAL_KEY on your specific workload. Performance improvements can vary based on the pattern of data access and the nature of the workload.

By addressing the specific challenge of last-page insert contention, OPTIMIZE_FOR_SEQUENTIAL_KEY offers a targeted solution for improving insert performance in scenarios where sequential key contention is a bottleneck, enhancing overall application throughput and scalability.