[[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 queries, aggregations, scripts, and when sorting:

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

PUT index_2/my_type/2
{
  "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": "doc['_index']" <4>
    }
  }
}
--------------------------
// AUTOSENSE

<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 (inline scripts must be <<enable-dynamic-scripting,enabled>> for this example to work)