Count distinct groups when using collapse

[dorian-marchal]

When collapsing results, the total number of hits (using track_total_hits) doesn't take collapsing into account, i.e. the total number of documents is returned, not the number of collapsed groups.

E.g. if I index 1508540 cars, from 116 brands, this query will return 1508540 in hits.total.value:

GET cars/_search?track_total_hits=true
{
  "collapse": {
    "field": "brand"
  }
}

If I want to count groups, I can use the cardinality aggregation.
This query will return the expected 116 in aggregations.brands_count.value but this is really slow for large indexes, event with a low precision_threshold.

GET cars/_search?size=0&track_total_hits=false
{
  "aggs": {
    "brands_count": {
      "cardinality": {
        "field": "brand",
        "precision_threshold": 150
      }
    }
  }
}

I could create a brands index, but in this example, I would need to sort collapsed results based on individual car features, so it would not help me.

I found some Github issues talking about this (here and here), but they are from 2017.
Do we have a way to count groups efficiently now? Is there some known workarounds?