---
title: _field_names field
description: The _field_names field used to index the names of every field in a document that contains any value other than null. This field was used by the exists...
url: https://www.elastic.co/elastic/docs-builder/docs/2454/reference/elasticsearch/mapping-reference/mapping-field-names-field
---

# _field_names field
The `_field_names` field used to index the names of every field in a document that contains any value other than `null`. This field was used by the [`exists`](https://www.elastic.co/elastic/docs-builder/docs/2454/reference/query-languages/query-dsl/query-dsl-exists-query) query to find documents that either have or don’t have any non-`null` value for a particular field.
Now the `_field_names` field only indexes the names of fields that have `doc_values` and `norms` disabled. For fields which have either `doc_values` or `norm` enabled the [`exists`](https://www.elastic.co/elastic/docs-builder/docs/2454/reference/query-languages/query-dsl/query-dsl-exists-query) query will still be available but will not use the `_field_names` field.

## Disabling `_field_names`

Disabling `_field_names` is no longer possible. It is now enabled by default because it no longer carries the index overhead it once did.
<note>
  Support for disabling `_field_names` has been removed. Using it on new indices will throw an error. Using it in pre-8.0 indices is still allowed but issues a deprecation warning.
</note>