[[mapping-id-field]]
=== `_id` field

Each document indexed is associated with a <<mapping-type-field,`_type`>> (see
<<mapping-type>>) and an <<mapping-id-field,`_id`>>.  The `_id` field is not
indexed as its value can be derived automatically from the
<<mapping-uid-field,`_uid`>> field.

The value of the `_id` field is accessible in certain queries (`term`,
`terms`, `match`, `query_string`, `simple_query_string`) and scripts, but
_not_ in aggregations or when sorting, where the <<mapping-uid-field,`_uid`>>
field should be used instead:

[source,js]
--------------------------
# Example documents
PUT my_index/my_type/1
{
  "text": "Document with ID 1"
}

PUT my_index/my_type/2
{
  "text": "Document with ID 2"
}

GET my_index/_search
{
  "query": {
    "terms": {
      "_id": [ "1", "2" ] <1>
    }
  },
  "script_fields": {
    "UID": {
      "script": "doc['_id']" <2>
    }
  }
}
--------------------------
// AUTOSENSE

<1> Querying on the `_id` field (also see the <<query-dsl-ids-query,`ids` query>>)
<2> Accessing the `_id` field in scripts (inline scripts must be <<enable-dynamic-scripting,enabled>> for this example to work)