Wildcard indexes

Starting with MongoDB 4.2, support has been added for wildcard indexes (https://docs.mongodb.com/master/core/index-wildcard/#wildcard-indexes). This feature is useful for situations where the index is either not yet available or is unknown. For situations where the field is known and well established, it's best to create a normal index. There are cases, however, where you have a subset of documents that contain a particular field otherwise lacking in other documents in the collection. You might also be in a situation where a field is added later, and where the DBA has not yet had a chance to create an index on the new field. In these cases, adding a wildcard index allows MongoDB to perform a query more efficiently.