coll = db.foo;
coll.drop();

for (let i = 0; i < 1000; i++) {
     coll.insertMany([{a: i,b:1}]);
}

db.runCommand({analyze: "foo", key: "a"});

db.adminCommand({setParameter: 1, planRankerMode: "heuristicCE"});
db.foo.aggregate([{$match:{a: {$size: 1}}}]).explain();

MongoServerError[Location9608701]: invalid MatchExpression passed to heuristic estimate: 4