[[mapping-field-names-field]]
=== `_field_names` field

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
<<query-dsl-exists-query,`exists`>> and <<query-dsl-missing-query,`missing`>>
queries 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, aggregations, and
scripts:

[source,js]
--------------------------
# Example documents
PUT my_index/my_type/1
{
  "title": "This is a document"
}

PUT my_index/my_type/1
{
  "title": "This is another document",
  "body": "This document has a body"
}

GET my_index/_search
{
  "query": {
    "terms": {
      "_field_names": [ "title" ] <1>
    }
  },
  "aggs": {
    "Field names": {
      "terms": {
        "field": "_field_names", <2>
        "size": 10
      }
    }
  },
  "script_fields": {
    "Field names": {
      "script": "doc['_field_names']" <3>
    }
  }
}

--------------------------
// AUTOSENSE

<1> Querying on the `_field_names` field (also see the <<query-dsl-exists-query,`exists`>> and <<query-dsl-missing-query,`missing`>> queries)
<2> Aggregating on the `_field_names` field
<3> Accessing the `_field_names` field in scripts (inline scripts must be <<enable-dynamic-scripting,enabled>> for this example to work)

