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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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44 changes: 9 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
Expand Up @@ -4,44 +4,18 @@
The `_field_names` field indexes the names of every field in a document that
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This line is no longer correct. Perhaps you can change it to "used to index" and explain that it is no longer required if the field has norms or doc values enabled.

contains any value other than `null`. This field is 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)

either have or don't have any non-+null+ value for fields that have
`doc_values` and `norms` disabled. Only fields which have `doc_values`
and `norms` disabled and are present in the document will have values
indexed into 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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