elasticsearch/docs/reference/query-languages/esql/esql-metadata-fields.md

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Metadata fields	https://www.elastic.co/guide/en/elasticsearch/reference/current/esql-metadata-fields.html

{{esql}} metadata fields [esql-metadata-fields]

{{esql}} can access metadata fields. The currently supported ones are:

• _index: the index to which the document belongs. The field is of the type keyword.
• _id: the source document’s ID. The field is of the type keyword.
• _version: the source document’s version. The field is of the type long.
• _ignored: the ignored source document fields. The field is of the type keyword.
• _score: when enabled, the final score assigned to each row matching an ES|QL query. Scoring will be updated when using full text search functions.

To enable the access to these fields, the FROM source command needs to be provided with a dedicated directive:

FROM index METADATA _index, _id

Metadata fields are only available if the source of the data is an index. Consequently, FROM is the only source commands that supports the METADATA directive.

Once enabled, these fields will be available to subsequent processing commands, just like other index fields:

FROM ul_logs, apps METADATA _index, _version
| WHERE id IN (13, 14) AND _version == 1
| EVAL key = CONCAT(_index, "_", TO_STR(id))
| SORT id, _index
| KEEP id, _index, _version, key

id:long	_index:keyword	_version:long	key:keyword
13	apps	1	apps_13
13	ul_logs	1	ul_logs_13
14	apps	1	apps_14
14	ul_logs	1	ul_logs_14

Similar to index fields, once an aggregation is performed, a metadata field will no longer be accessible to subsequent commands, unless used as a grouping field:

FROM employees METADATA _index, _id
| STATS max = MAX(emp_no) BY _index

max:integer	_index:keyword
10100	employees