Hello @Harsh_Chaudhary, Welcome to the MongoDB community forum,
I hope you know the difference between a covered query and an indexed query (improves the performance).
If not then refer to this explanation about the covered query:
A covered query is a query that can be satisfied entirely using an index and does not have to examine any documents. An index covers a query when all of the following apply:
- all the fields in the query are part of an index, and
- all the fields returned in the results are in the same index.
- no fields in the query are equal to
null(i.e. {"field" : null} or {"field" : {$eq : null}}).
Specifically for the multi-key index, each array element is indexed separately. So, it does not guarantee that the index alone can satisfy the entire query without examining the documents.
I would suggest you play with explain command, also use MongoDB Compass to visualize and understand the execution and index use by clicking on the Explain button.