Document metadata fields
Each document has metadata associated with it, such as the _index and _id metadata fields. The behavior of some of these metadata fields can be customized when a mapping is created.
_source- The original JSON representing the body of the document.
_size- The size of the
_sourcefield in bytes, provided by themapper-sizeplugin.
_doc_count- A custom field used for storing doc counts when a document represents pre-aggregated data.
_field_names- All fields in the document which contain non-null values.
_ignored- All fields in the document that have been ignored at index time because of
ignore_malformed.
_routing- A custom routing value which routes a document to a particular shard.