The _field_names field indexes the names of every field in a document that contains any value other than null. This field is used by the exists query to find documents that either have or don’t have any non-null value for a particular field.

The value of the _field_name field is accessible in queries:

# Example documents
PUT my_index/my_type/1
{
  "title": "This is a document"
}

PUT my_index/my_type/2?refresh=true
{
  "title": "This is another document",
  "body": "This document has a body"
}

GET my_index/_search
{
  "query": {
    "terms": {
      "_field_names": [ "title" ] 
    }
  }
}