[[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`>> query to find documents that
either have or don't have any non-+null+ value for a particular field.

The value of the `_field_names` field is accessible in queries:

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

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

GET my_index/_search
{
  "query": {
    "terms": {
      "_field_names": [ "title" ] <1>
    }
  }
}

--------------------------
// CONSOLE

<1> Querying on the `_field_names` field (also see the <<query-dsl-exists-query,`exists`>> query)


==== Disabling `_field_names`

Because `_field_names` introduce some index-time overhead, you might want to
disable this field if you want to optimize for indexing speed and do not need
`exists` queries.

[source,js]
--------------------------------------------------
PUT tweets
{
  "mappings": {
    "_doc": {
      "_field_names": {
        "enabled": false
      }
    }
  }
}
--------------------------------------------------
// CONSOLE