What is the primary use of indexes in MongoDB?

The primary use of indexes in MongoDB is to speed up queries and reduce the resources required to retrieve data from the database. When an index is created on a specific field or fields within a collection, MongoDB can quickly locate the documents that match the query criteria without having to scan the entire collection. This greatly improves query performance, especially for large datasets, as it allows the database engine to take advantage of the index structure to find relevant data more efficiently.

Indexes work similarly to an index in a book, where you can find specific topics without having to read every page. In MongoDB, common types of indexes include single-field indexes, compound indexes, text indexes, and geospatial indexes, each designed to optimize query performance for specific use cases.

While other options touch on different aspects of data handling, they do not address the primary function of indexes. The decrease in document size refers more to optimization techniques like data compression but doesn't relate to indexing. Backups are separate mechanisms used for data recovery, and establishing data relationships pertains more to schema design rather than indexing, which is focused specifically on enhancing query speed and efficiency.