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Field names

The _field_names field indexes field names that contain non-null values. This enables the use of the exists query, which can identify documents that either have or do not have non-null values for a specified field.

However, _field_names only indexes field names when both doc_values and norms are disabled. If either doc_values or norms are enabled, then the exists query still functions but will not rely on the _field_names field.

Mapping example

{
    "mappings": {
       "_field_names": {
        "enabled": "true"
      },
    "properties": {
      },
      "title": {
        "type": "text",
        "doc_values": false,
        "norms": false
      },
      "description": {
        "type": "text",
        "doc_values": true,
        "norms": false
      },
      "price": {
        "type": "float",
        "doc_values": false,
        "norms": true
      }
    }
  }
}