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

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[[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.
==== 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` be adding the following to the mappings:
[source,js]
--------------------------------------------------
PUT tweets
{
"mappings": {
"_field_names": {
"enabled": false
}
}
}
--------------------------------------------------
// CONSOLE