Null valued fields are treated as existing

OpenSearch
1

Versions: 3.3

Describe the issue: I created a index that has _field_names enabled and has a field with a null_value configured. The Field names documentation says that

“The _field_names field indexes field names that contain non-null values”

So I indexed a null value, but when searching with the exists clause I get hits with null values.

This shouldn’t happen, it’s to be expected that configured null_valued fields retain the same meaning as they were really null, thus being treated as non-existent as the documentation suggest.

Minimal Reproducible Example:

PUT example
{
  "mappings": {
    "_field_names": {
      "enabled": true
    },
    "properties": {
      "exampleField": {
        "type": "long",
        "null_value": 0
      }
    }
  }
}

PUT example/_doc/1
{
  "exampleField": null
}

POST example/_search
{
  "query": {
    "bool": {
      "must": [
        {"exists": {"field": "exampleField"}}
      ]
    }
  }
}

Executing this example you see that the document will be returned even though it’s value is null, contradicting the cited expected behavior.