What is the ideal field order for a compound index?

The ideal field order for a compound index in MongoDB is based on how queries will utilize the index. The best practice is to place equality fields first, followed by sort fields, and lastly range fields. This order is beneficial because:

1. Equality Fields: These are typically used to filter the documents. By placing equality fields first in the compound index, MongoDB can quickly narrow down the potential matches before considering other criteria.

2. Sort Fields: After filtering with equality checks, the database may also require sorting results based on specific fields. By positioning sort fields next in the index order, it allows MongoDB to use the same index for both filtering and sorting, which improves performance by eliminating the need for extra sorting operations post-retrieval.

3. Range Fields: Finally, the range fields are placed last. These fields account for operations like greater than, less than, and not equal comparisons. Since range queries typically result in a variable number of results, placing them last allows MongoDB to optimize the use of the preceding equality and sort information first.

This sequencing leverages the way the query engine normally looks at indexes, ensuring efficiency in retrieval and processing. By following this order, MongoDB can maximize the benefits of