For compound indexes, this rule of thumb is helpful in deciding the order of fields in the index:
**Covered queries return results from an index directly without having to access the source documents, and are therefore very efficient.**
For a query to be covered all the fields needed for filtering, sorting and/or being returned to the client must be present in an index. To determine whether a query is a covered query, use the explain() method. If the explain() output displays totalDocsExamined as 0, this shows the query is covered by an index. Read more in the documentation for explain results.
A common gotcha when trying to achieve covered queries is that the _id field is always returned by default. You need to explicitly exclude it from query results, or add it to the index.
In sharded clusters, MongoDB internally needs to access the fields of the shard key. This means covered queries are only possible when the shard key is part of the index. It is usually a good idea to do this anyway.
Queries on fields with a small number of unique values (low cardinality) can return large result sets. Compound indexes may include fields with low cardinality, but the value of the combined fields should exhibit high cardinality.
Reduce the size and performance overhead of indexes by only including documents that will be accessed through the index. For example, create a partial index on the orderID field that only includes order documents with an orderStatus of "In progress", or only indexes the emailAddress field for documents where it exists.
sparse Index