Request for index selection based solely on the partialFilterExpression.

Currently, the query optimizer decides on an index based on the match and sort arguments of a query. With partial indexes, it will also consider the partialFilterExpression, but as implemented, it can still miss an index that would be performant on the query.

Here's an example where the current implementation is sub-optimal:

db.foo.dropIndex();
db.foo.createIndex( { a : 1 }, { partialFilterExpression : { b : { $gte : 5 } } } );
for (i=1; i<=10; i++) { db.foo.insertOne( { a : i, b : i } ) };
db.foo.find( { b : { $gte : 5 } }, { _id : 0, a : 1 } ).explain()

Desired result: The query uses the index, and it is a covered query
Current result: the query performs a collection scan
Variation: db.foo.find( { a : { $exists: true }, b : { $gte : 5 } }, { _id : 0, a : 1 } ).explain() results in an index scan, but does not cover the query even though the index could cover this query.