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

[[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
<<query-dsl-exists-query,`exists`>> 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 <<query-dsl-exists-query,`exists`>> query will still
be available but will not use the `_field_names` field.

[[disable-field-names]]
==== 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.