A compound wildcard index(CWI), {a: 1, b: 1, "sub.$**": 1, c: 1}, should be able to use on queries on only the non-wildcard prefix of this index, e.g. query: {a: 1, b: 1}.

This requires:

1. CWI can generate index keys for document w/o any wildcard field, e.g. {a: 1, b: 1}
2. Wildcard expanding should expand a wildcard index entry to an IndexEntry even though no wildcard field is in the query.
3. Fields after such wildcard field should not support non-blocking sort. (In the example above, field 'c' cannot support non-blocking sort)