When would you use a multikey index?

Using a multikey index is particularly beneficial when you have fields in your MongoDB documents that contain arrays. In scenarios where you need to index array fields, a multikey index allows MongoDB to create an index for each value in the array for a document. This means you can efficiently query documents based on the individual elements of these arrays.

For example, if you have a collection of documents representing users and each user document has an array of "hobbies," a multikey index on the "hobbies" field would allow you to quickly find all users who have a specific hobby, even if multiple users have the same hobby or if a user has multiple hobbies.

This indexing capability is a key feature of MongoDB that enhances query performance when dealing with array data structures, facilitating complex queries that would be less efficient without the multikey index.