Cardinality agg on constant_keyword fields returning zero in 8.9+

[ebeahan]

Elasticsearch Version

8.9.0+

Installed Plugins

No response

Java Version

bundled

OS Version

all

Problem Description

Beginning in 8.9.0, observing cardinality aggs returning zero against constant_keyword mapped fields.

Steps to Reproduce

Performed simple setup of an index with constant_keyword mapping for field test in 8.8.2 and 8.9.0:

PUT test-index
{
  "mappings": {
    "properties": {
      "test": {
        "type": "constant_keyword"
      }
    }
  }
}

PUT test-index/_bulk
{ "index" : { "_index" : "test-index", "_id" : "1" } }
{ "test" : "value1" }



GET test-index/_search
{
  "aggs": {
    "0": {
      "cardinality": {
        "field": "test"
      }
    }
  },
  "size": 0
}

8.8.2 result

{
  "took": 1,
  "timed_out": false,
  "_shards": {
    "total": 1,
    "successful": 1,
    "skipped": 0,
    "failed": 0
  },
  "hits": {
    "total": {
      "value": 1,
      "relation": "eq"
    },
    "max_score": null,
    "hits": []
  },
  "aggregations": {
    "0": {
      "value": 1 <= expected
    }
  }
}

8.9.0 result

{
  "took": 2,
  "timed_out": false,
  "_shards": {
    "total": 1,
    "successful": 1,
    "skipped": 0,
    "failed": 0
  },
  "hits": {
    "total": {
      "value": 1,
      "relation": "eq"
    },
    "max_score": null,
    "hits": []
  },
  "aggregations": {
    "0": {
      "value": 0 <= unexpected
    }
  }
}

Also see a zero value when querying constant_keyword values across indices:

# `test` already mapped as `constant_keyword` for `test-index-*`
PUT test-index/_bulk
{ "index" : { "_index" : "test-index-0001", "_id" : "1" } }
{ "test" : "value1" }
{ "index" : { "_index" : "test-index-0002", "_id" : "2" } }
{ "test" : "value2" }
{ "index" : { "_index" : "test-index-0003", "_id" : "3" } }
{ "test" : "value3" }
{ "index" : { "_index" : "test-index-0004", "_id" : "4" } }
{ "test" : "value4" }
{ "index" : { "_index" : "test-index-0005", "_id" : "5" } }
{ "test" : "value5" }



GET test-index-*/_search
{
  "aggs": {
    "0": {
      "cardinality": {
        "field": "test"
      }
    }
  },
  "size": 0
}

Logs (if relevant)

No response

[elasticsearchmachine]

Pinging @elastic/es-analytics-geo (Team:Analytics)

[kkrik-es]

This is due to #92060, I think. indexTerms is null for a const_term field, returning LeafBucketCollector.NO_OP_COLLECTOR. I'll look into adding a check for this case.