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:

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"] 
    }
  },
  "aggs": {
    "indices": {
      "terms": {
        "field": "_index", 
        "size": 10
      }
    }
  },
  "sort": [
    {
      "_index": { 
        "order": "asc"
      }
    }
  ],
  "script_fields": {
    "index_name": {
      "script": {
        "lang": "painless",
        "source": "doc['_index']" 
      }
    }
  }
}

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.