OpenSearch/docs/reference/mapping/fields/index-field.asciidoc

[[mapping-index-field]]
=== `_index` field

When performing queries across multiple indexes, it is sometimes desirable to
add query clauses that are associated with documents of only certain indexes.
The `_index` field allows matching on the index a document was indexed into.
Its value is accessible in `term`, or `terms` queries, aggregations,
scripts, and when sorting:

NOTE: The `_index` is exposed as a virtual field -- it is not added to the
Lucene index as a real field.  This means that you can use the `_index` field
in a `term` or `terms` query (or any query that is rewritten to a `term`
query, such as the `match`,  `query_string` or `simple_query_string` query),
but it does not support `prefix`, `wildcard`, `regexp`, or `fuzzy` queries.

[source,console]
--------------------------
# Example documents
PUT index_1/_doc/1
{
  "text": "Document in index 1"
}

PUT index_2/_doc/2?refresh=true
{
  "text": "Document in index 2"
}

GET index_1,index_2/_search
{
  "query": {
    "terms": {
      "_index": ["index_1", "index_2"] <1>
    }
  },
  "aggs": {
    "indices": {
      "terms": {
        "field": "_index", <2>
        "size": 10
      }
    }
  },
  "sort": [
    {
      "_index": { <3>
        "order": "asc"
      }
    }
  ],
  "script_fields": {
    "index_name": {
      "script": {
        "lang": "painless",
        "source": "doc['_index']" <4>
      }
    }
  }
}
--------------------------

<1> Querying on the `_index` field
<2> Aggregating on the `_index` field
<3> Sorting on the `_index` field
<4> Accessing the `_index` field in scripts