The index to use for the aggregation. The index is on the initial collection / view against which the aggregation is run.
.typecons .Nullable!(vibe.data.bson.Bson) hint ;
The hint does not apply to $lookup and $graphLookup stages.
Specify the index either by the index name as a string or the index key pattern. If specified, then the query system will only consider plans using the hinted index.