[[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>>. The `_id` can either be assigned at
indexing time, or a unique `_id` can be generated by {es}. This field is not
configurable in the mappings.

The value of the `_id` field is accessible in queries such as `term`,
`terms`, `match`, and `query_string`.

[source,console]
--------------------------
# Example documents
PUT my-index-000001/_doc/1
{
  "text": "Document with ID 1"
}

PUT my-index-000001/_doc/2?refresh=true
{
  "text": "Document with ID 2"
}

GET my-index-000001/_search
{
  "query": {
    "terms": {
      "_id": [ "1", "2" ] <1>
    }
  }
}
--------------------------

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

The `_id` field is restricted from use in aggregations, sorting, and scripting.
In case sorting or aggregating on the `_id` field is required, it is advised to
duplicate the content of the `_id` field into another field that has
`doc_values` enabled.

[NOTE]
==================================================
`_id` is limited to 512 bytes in size and larger values will be rejected.
==================================================