Currently, users may hint the use of a wildcard index, but that does not avoid multi-planning because the wildcard index is presented as many different sub-indexes to the query planner. It would be helpful to allow hints for a specific field of a wildcard index.

The example provided by alexander.ignatyev@mongodb.com, with a compound wildcard index, is:

db.cwi.createIndex({"a.$**": 1, "b": 1});
const docs = [{a: {x: 1, y : 2}, b: 1}, {a: {x: 2, y: 3}, b: 1}];
db.cwi.insertMany(docs);
const explain = db.cwi.find({"a.x": 1, b: 1}).explain("executionStats");
explain.queryPlanner.winningPlan.inputStage.keyPattern;

The above will print:

{ '$_path': 1, 'a.x': 1, b: 1 }

But right now, this cannot be supplied as a hint.