[[mapping-uid-field]]
=== `_uid` field

deprecated[6.0.0, Now that types have been removed, documents are uniquely
identified by their `_id` and the `_uid` field has only been kept as a view
over the `_id` field for backward compatibility.]

Each document indexed is associated with a <<mapping-type-field,`_type`>> (see
<<mapping-type>>) and an <<mapping-id-field,`_id`>>.  These values are
combined as `{type}#{id}` and indexed as the `_uid` field.

The value of the `_uid` field is accessible in queries, aggregations, scripts,
and when sorting:

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

PUT my_index/my_type/2?refresh=true
{
  "text": "Document with ID 2"
}
--------------------------
// CONSOLE

[source,js]
--------------------------
GET my_index/_search
{
  "query": {
    "terms": {
      "_uid": [ "my_type#1", "my_type#2" ] <1>
    }
  },
  "aggs": {
    "UIDs": {
      "terms": {
        "field": "_uid", <2>
        "size": 10
      }
    }
  },
  "sort": [
    {
      "_uid": { <3>
        "order": "desc"
      }
    }
  ],
  "script_fields": {
    "UID": {
      "script": {
         "lang": "painless",
         "inline": "doc['_uid']" <4>
      }
    }
  }
}
--------------------------
// CONSOLE
// TEST[continued]
// TEST[warning:Fielddata access on the _uid field is deprecated, use _id instead]

<1> Querying on the `_uid` field (also see the <<query-dsl-ids-query,`ids` query>>)
<2> Aggregating on the `_uid` field
<3> Sorting on the `_uid` field
<4> Accessing the `_uid` field in scripts