[[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 certain queries and aggregations, and when sorting
or scripting:

[source,console]
--------------------------
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

The `_index` field is exposed virtually -- 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), as well as `prefix`
and `wildcard` queries. However, it does not support `regexp` and `fuzzy`
queries.

Queries on the `_index` field accept index aliases in addition to concrete
index names.

NOTE: When specifying a remote index name such as `cluster_1:index_3`, the
query must contain the separator character `:`. For example, a `wildcard` query
on `cluster_*:index_3` would match documents from the remote index. However, a
query on `cluster*index_1` is only matched against local indices, since no
separator is present. This behavior aligns with the usual resolution rules for
remote index names.
