What is a multikey index?

A multikey index is specifically designed to handle situations where one of the indexed fields contains an array. In MongoDB, when a field within a document is an array, that field cannot be indexed in the traditional manner since there are multiple values that need to be accounted for. A multikey index allows MongoDB to create an index that includes each element of the array as an individual index key.

This type of indexing is particularly useful because it enables efficient queries on documents that contain array fields. For example, if you have a collection of documents where one of the fields is an array of tags, a multikey index on that field allows for quick searches based on the tags present in any of the documents.

The other options do not capture the essence of what a multikey index is. An index with a single unique field describes a unique index rather than a multikey. An automatically created index refers to default behaviors in MongoDB, which doesn't specifically imply it being a multikey index. Moreover, an index that cannot be queried would not serve its purpose since the primary function of any index is to facilitate efficient data retrieval.