_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
exists query to find documents that
either have or don’t have any non-
null value for a particular field.
_field_names field only indexes the names of fields that have
norms disabled. For fields which have either
norm enabled the
exists query will still
be available but will not use the
_field_names is no longer possible. It is now enabled by default
because it no longer carries the index overhead it once did.
Support for disabling
_field_names has been removed. Using it on new
indices will throw an error. Using it in pre-8.0 indices is still allowed but
issues a deprecation warning.