AX / Dynamics 365 For Finance and Operations blog

In Dynamics AX and Dynamics 365 Finance and Operations (D365FO), you may encounter situations where you need to force a query to use a specific index for optimal performance. This can be useful for improving query execution times, especially when dealing with large datasets or complex queries. This guide will show you how to use index hints to achieve this by forcing a SELECT statement to use a specific index.

Enabling Index Hints

In D365FO, you can use the allowIndexHint() method and the index hint clause to guide the database on which index to use for a query. Here is how you can do it:

Example Code

InventTrans inventTrans;

// Enable index hints on the query
inventTrans.allowIndexHint(true); // Use this method to enable index hinting

// Execute a SELECT query, specifying the desired index
select inventTrans
    index hint StatusItemIdx // Specify the index you want the query to use
    where inventTrans.StatusIssue == StatusIssue::OnOrder;

Explanation of the Code

1. allowIndexHint(true)
   This method call enables index hints for the inventTrans table. By default, index hinting is disabled, and this method is required to activate it.
2. index hint StatusItemIdx
   This clause forces the query to use the specified index (StatusItemIdx) when executing the SELECT statement. This can be beneficial for performance if you know that the specified index provides faster access to the data for your specific conditions.

When to Use Index Hints

• Performance Optimization: If the query optimizer chooses an inefficient index or performs a full table scan, specifying an index may significantly improve query speed.
• Query Troubleshooting: When troubleshooting slow queries, forcing a particular index can help identify if indexing is the source of the issue.
• Control over Query Execution: You gain greater control over query execution plans, which is useful in certain complex scenarios.

Important Considerations

1. Understand the Indexes
   Before using index hints, you should be familiar with the available indexes on the table and the data distribution. Forcing an inappropriate index may degrade performance rather than improve it.
2. Testing and Validation
   Always test the performance impact of using index hints in a non-production environment first. What works for one query may not work well for others.
3. Maintenance and Schema Changes
   Indexes can change over time as the schema evolves. Be mindful that hardcoding index names can lead to issues if the index is altered or removed.

Practical Use Case

Imagine you have a table InventTrans with many records, and you frequently query the table using StatusIssue. Without an index hint, the database engine might choose an index that is less optimal or perform a full table scan. By specifying StatusItemIdx as the index to use, you can potentially speed up query execution by narrowing down the data access path.

Conclusion

Using index hints in D365FO can provide a powerful way to control query execution and optimize performance when needed. However, it should be done with caution and a deep understanding of the underlying data structure and indexes. Properly applied, this technique can lead to significant performance improvements in your queries and overall system responsiveness.