What is a compound index?

A compound index is specifically designed to provide efficient querying capabilities for multiple fields within a single document. This type of index allows the MongoDB query optimizer to quickly locate documents based on the values of multiple fields, which can significantly improve query performance when searching for documents by combinations of these fields.

When an index includes multiple fields, it can be used in several different querying scenarios. For example, if you create a compound index on the fields "firstName" and "lastName," MongoDB can use this index to efficiently execute queries that filter on both fields or on either field. This capability is particularly beneficial when queries involve both fields in combination, enhancing the overall speed and effectiveness of data retrieval.

The other choices describe different indexing concepts. An index that contains references to a single field is a single-field index, which is less versatile than a compound index. An index that references multiple collections does not fit the definition of how indexes function in MongoDB, as indexes are designed to operate within the context of a single collection. The final option, while indicating that indexes are used for optimizing query performance, does not capture the specific nature of a compound index focused on multiple fields.