Boosting Query Performance with JSON Indexes in SQL Server 2025

SQL Server 2025 introduces a native JSON index, a game-changing feature for developers and database administrators working with semi-structured data. As JSON becomes the go-to format for modern data exchange and storage, especially in RESTful applications, the need for efficient querying within SQL Server has never been greater. 

Traditional methods like JSON_VALUE often resulted in slow, full table scans, making real-time analytics and high-performance APIs challenging to maintain. With the JSON index, SQL Server now offers a direct, optimized path to querying JSON data.

Understanding the JSON Index

The JSON index is specifically designed to target certain paths within a JSON column, allowing for rapid data retrieval without scanning every row. With a simple declaration, you can direct SQL Server to build an index for the JSON keys your application queries most often:

CREATE JSON INDEX ix_json ON dbo.mytable(myjsoncolumn) FOR ('$.some.path', '$.another.path');

This targeted approach means only the relevant JSON paths are indexed, saving space and boosting query efficiency.

Essential Requirements and Limitations

While the JSON index is powerful, there are several requirements to consider before implementing it:

• Available exclusively in SQL Server 2025 (on-premises, preview release).
  
  
• Target tables must include a clustered primary key.
  
  
• JSON indexes are not available for views, table-valued variables, or memory-optimized tables.
  
  
• The indexed column must use the native JSON data type not VARCHAR(MAX) Copy or computed columns.
  
  
• Each column can have one JSON index, but multiple JSON paths can be included; up to 249 JSON indexes per table are allowed.
  
  
• Current limitations include no data compression or index hints.

Planning your data model and table structure around these constraints is key to unlocking the full benefits of the JSON index.

Hands-On: Creating and Using a JSON Index

Setting up a JSON index is straightforward. The process involves:

• Creating a table with a JSON column and a clustered primary key.
• Populating the table with JSON data, typically using the FOR JSON clause.
• Defining the JSON index with the JSON paths your queries will target, such as '$.Name.FirstName' Copy.

It's important to avoid overlapping paths (like both '$.Name' Copy and '$.Name.FirstName' Copy), as this can cause errors. Internally, SQL Server maintains the JSON index in a hidden table, which should be considered when estimating storage needs.

Performance Gains: Real-World Results

The impact on query performance is significant. When searching for specific values using functions like JSON_CONTAINS, the engine can skip full table scans and perform direct seeks on the indexed JSON paths. In one benchmark, removing the index resulted in over 101,000 page reads, while with the JSON index, reads dropped to just 48 on the index and about 17,000 on the main table—demonstrating major resource savings for selective queries.

Current Limitations and Operator Support

Despite its promise, the JSON index in the preview release doesn't support all JSON operators as expected. For example, while documentation claims support for JSON_VALUE, real-world tests show mixed results. Nevertheless, feedback from the community and ongoing development suggest broader compatibility is on the horizon.

When and How to Use JSON Indexes

• Best suited for queries retrieving small, selective result sets based on JSON path values.
  
  
• Offers less advantage for bulk queries that scan large portions of the table, due to required lookups.
  
  
• Consider the storage impact of hidden internal index tables during capacity planning.

Takeaway: A Step Forward for Semi-Structured Data

SQL Server 2025’s native JSON index marks a significant step forward for performance tuning and semi-structured data management. By aligning table designs and queries with its strengths and limitations, organizations can achieve faster, more scalable solutions for JSON-heavy workloads. As SQL Server continues to evolve, expect even more robust support for JSON indexing in future updates.

Source: MSSQLTips.com