Why is the sort order of field values in an index significant?

The significance of the sort order of field values in an index primarily lies in its ability to avoid in-memory sorts, which can be performance-intensive. When the database engine executes a query that includes sorting criteria, if the data is already stored in the correct order thanks to the index, MongoDB can retrieve the results efficiently without requiring any additional sorting operations. This leads to faster query execution times, reduced memory usage, and overall improved query performance.

On the other hand, if the data were not indexed in the order required for the query, MongoDB would need to perform an in-memory sort on the retrieved documents, which can consume significant system resources and time, especially with large datasets. Hence, maintaining an appropriate sort order in your indexes is crucial for optimizing query performance by enabling direct, ordered access to the relevant data without additional overhead.