Fast Vector Highlighter does no work with nested field as target

[pbvahlst]

Elasticsearch version (bin/elasticsearch --version): 7.15.2

Plugins installed: []

JVM version (java -version): the one included in ES

OS version (uname -a if on a Unix-like system): Windows 10

Description of the problem including expected versus actual behavior:
When using a nested field as target for Fast Vector Highligter no result is produced. (This works fine with the Unified Highlighter).
The problem seems to be that fvh tries to read the target field literally from the source and as this would not be present in the source
it assumes that the field is absent. If the nested field has store: true set everything works as expected as it then reads the stored field instead of visiting the source-object.

The reason for using a nested field as target is that the root-field is a date-field and we then have multiple nested fields with different versions of the date-string with with_positions_offsets enabled for term_vector.

Steps to reproduce:
make a search like the following with and without store:true enabled in the mapping of the the nested target field (date.text).

{
  "query": {
    "query_string": {
      "query": "2000-10-10",
      "fields": [ "date.text" ],
      "quote_field_suffix": ".exact"
    }
  },
  "highlight": {
    "fields": {
      "date.text": {
        "matched_fields": [ "date.text", "date.text.exact" ],
        "type": "fvh"
      }
    }
  }
}

[elasticmachine]

Pinging @elastic/es-search (Team:Search)

[Daantie]

I'm running into this problem when trying to upgrade Elasticsearch 7.11 to 7.16. A lot of highlighting stuff in our project breaks due to this. The problem is not only for nested fields, or any particular query (like in #82458), but to subfields in general. I have done some investigation and it started in 7.14 and is still the same in 8.0.0.

I've reproduced it with these steps:

PUT simple-highlight-test
{
  "mappings": {
    "properties": {
      "text": {
        "type": "text",
        "analyzer": "standard",
        "term_vector": "with_positions_offsets",
        "fields": {
          "subfield": {
            "type": "text",
            "term_vector": "with_positions_offsets",
            "analyzer": "standard"
          }
        }
      }
    }
  }
}

POST simple-highlight-test/_doc
{
  "text": "some random text"
}

GET simple-highlight-test/_search
{
  "from": 0,
  "size": 10,
  "query": {
    "match": {
      "text.subfield": {
        "query": "random"
      }
    }
  },
  "highlight": {
    "fields": [
      {
        "text.subfield": {
          "type": "fvh"
        }
      }
    ]
  }
}

Like mentioned by OP adding "store": true to the subfield will make it work, but this obviously results in larger index size. Plain and unified highlighters work fine without store true though.

@cbuescher do you know if this is getting any attention? Or will be getting any attention any time soon?