Where should fields that are not queried but are used in the final projection be placed in an index?

When designing an index in MongoDB, the order of fields within the index can significantly affect performance, especially for queries and projections. Fields that are not queried but are included in the final projection should be placed at the end of the index.

The rationale behind this is efficiency in how MongoDB uses indexes during query execution. Fields that are part of the query conditions (the "query keys") should be given priority in the index structure since they directly influence the filtering of documents. Placing fields that are merely projected at the end of the index allows MongoDB to quickly evaluate the query against the indexed fields first, and then easily access the additional non-queried fields for the projection without affecting the performance of retrieving the actual query results.

In this scenario, position matters significantly; having non-queried fields at the beginning or in the middle of the index could lead to unnecessary complexity and overhead in the indexing process. This arrangement can slow down query performance, as it forces MongoDB to process more data than necessary for fulfilling the query.

Consequently, placing those fields at the end of the index is a best practice that optimizes query execution while still allowing for easy access to the projected fields.