What are compound indexes in MongoDB?

Compound indexes in MongoDB are designed to improve query performance by allowing for indexing on multiple fields simultaneously. This means that when you create a compound index, you specify two or more fields that will be used together when executing queries or retrieving data. The advantage of compound indexes is that they can efficiently support queries that filter on multiple fields by leveraging the index to quickly locate the necessary documents.

For instance, if you frequently query a collection based on both the "lastName" and "firstName" fields, creating a compound index on these two fields will significantly speed up those queries compared to using separate single-field indexes. This is especially useful for queries that combine filtering and sorting on these fields, as the compound index can also be used to optimize sorting operations.

In contrast, indexes that contain only a single field do not provide this level of efficiency for queries involving more than one field. Also, while it's true that indexes can sometimes be created based on usage patterns, this is not a defining characteristic of compound indexes specifically. Lastly, indexes that cannot be used in queries would not serve their intended purpose, as the fundamental role of an index is to speed up query execution. Therefore, the defining feature of compound indexes being able to index multiple fields makes the answer about