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[DOCS] Rewords _field_names documentation #31029

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Jun 4, 2018
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[DOCS] Rewords _field_names documentation #31029

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[DOCS] Rewords _field_names documentation
colings86 Jun 1, 2018
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Update wording following review
colings86 Jun 1, 2018
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46 changes: 11 additions & 35 deletions docs/reference/mapping/fields/field-names-field.asciidoc
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Original file line number Diff line number Diff line change
@@ -1,47 +1,23 @@
[[mapping-field-names-field]]
=== `_field_names` field

The `_field_names` field indexes the names of every field in a document that
contains any value other than `null`. This field is used by the
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.

The value of the `_field_names` field is accessible in queries:

[source,js]
--------------------------
# Example documents
PUT my_index/_doc/1
{
"title": "This is a document"
}

PUT my_index/_doc/2?refresh=true
{
"title": "This is another document",
"body": "This document has a body"
}

GET my_index/_search
{
"query": {
"terms": {
"_field_names": [ "title" ] <1>
}
}
}

--------------------------
// CONSOLE

<1> Querying on the `_field_names` field (also see the <<query-dsl-exists-query,`exists`>> query)

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`

Because `_field_names` introduce some index-time overhead, you might want to
disable this field if you want to optimize for indexing speed and do not need
`exists` queries.
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]
--------------------------------------------------
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