What functionality does having a unique index on a field provide?

Having a unique index on a field ensures that all values in that indexed field are distinct across the documents in a collection. This means that no two documents can have the same value for that particular field, which enforces data integrity and uniqueness within the dataset.

For instance, if you have an indexed field for email addresses in a user collection, with a unique index established on that field, the database will not allow the insertion of another record with an identical email address. This is particularly useful for fields like identifiers, usernames, or any other attribute where uniqueness is a requirement for the application logic.

The other options do not accurately describe the behavior of a unique index. For example, allowing multiple records with the same value directly contradicts the purpose of a unique index. Optimizing sorting operations and combining multiple fields into a single index pertain to different indexing strategies and functionalities that are unrelated to the uniqueness constraint provided by a unique index.