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

Each document has an `_id` that uniquely identifies it, which is indexed
so that documents can be looked up either with the <<docs-get,GET API>> or the
<<query-dsl-ids-query,`ids` query>>.

NOTE: This was not the case with pre-6.0 indices due to the fact that they
supported multiple types, so the `_type` and `_id` were merged into a composite
primary key called `_uid`.

The value of the `_id` field is accessible in certain queries (`term`,
`terms`, `match`, `query_string`, `simple_query_string`).

[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"
}

GET my_index/_search
{
  "query": {
    "terms": {
      "_id": [ "1", "2" ] <1>
    }
  }
}
--------------------------
// CONSOLE

<1> Querying on the `_id` field (also see the <<query-dsl-ids-query,`ids` query>>)

The value of the `_id` field is also accessible in aggregations or for sorting,
but doing so is discouraged as it requires to load a lot of data in memory. In
case sorting or aggregating on the `_id` field is required, it is advised to
duplicate the content of the `_id` field in another field that has `doc_values`
enabled.