[[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 often not necessary because it no longer
carries the index overhead it once did. If you have a lot of fields
which have `doc_values` and `norms` disabled and you do not need to
execute `exists` queries using those fields you might want to disable
`_field_names` by adding the following to the mappings:

[source,console]
--------------------------------------------------
PUT tweets
{
  "mappings": {
    "_field_names": {
      "enabled": false
    }
  }
}
--------------------------------------------------
