elasticsearch/docs/reference/mapping/fields/field-names-field.asciidoc

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

The `_field_names` field indexes the field names of a document, which can later
be used to search for documents based on the fields that they contain typically
using the `exists` and `missing` filters.

`_field_names` is indexed by default for indices that have been created after
Elasticsearch 1.3.0.