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
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 exists query will still
be available but will not use the _field_names field.
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:
PUT tweets
{
"mappings": {
"_field_names": {
"enabled": false
}
}
}