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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.

Identity metadata fields

`_index`	The index to which the document belongs.
`_id`	The document’s ID.

Document source metadata fields

_source: The original JSON representing the body of the document.
_size: The size of the _source field in bytes, provided by the mapper-size plugin.

Doc count metadata field

_doc_count: A custom field used for storing doc counts when a document represents pre-aggregated data.

Indexing metadata fields

_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 metadata field

_routing: A custom routing value which routes a document to a particular shard.

Other metadata field

_meta: Application specific metadata.
_tier: The current data tier preference of the index to which the document belongs.

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https://www.elastic.co/guide/en/elasticsearch/reference/8.17/mapping-fields.html