OpenSearch/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.
Index field names of documents. The `exists` and `missing` filters need to merge postings lists of all existing terms, which can be very costly, especially on high-cardinality fields. This commit indexes the field names of a document under `_field_names` and reuses it to speed up the `exists` and `missing` filters. This is only enabled for indices that are created on or after Elasticsearch 1.3.0. Close #5659 2014-05-20 11:34:00 -04:00			`[[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.