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ID

Each document in OpenSearch has a unique _id field. This field is indexed, allowing you to retrieve documents using the GET API or the ids query.

If you do not provide an _id value, then OpenSearch automatically generates one for the document.

The following example request creates an index named test-index1 and adds two documents with different _id values:

PUT test-index1/_doc/1
{
  "text": "Document with ID 1"
}

PUT test-index1/_doc/2?refresh=true
{
  "text": "Document with ID 2"
}

You can then query the documents using the _id field, as shown in the following example request:

GET test-index1/_search
{
  "query": {
    "terms": {
      "_id": ["1", "2"]
    }
  }
}

The response returns both documents with _id values of 1 and 2:

{
  "took": 10,
  "timed_out": false,
  "_shards": {
    "total": 1,
    "successful": 1,
    "skipped": 0,
    "failed": 0
  },
  "hits": {
    "total": {
      "value": 2,
      "relation": "eq"
    },
    "max_score": 1,
    "hits": [
      {
        "_index": "test-index1",
        "_id": "1",
        "_score": 1,
        "_source": {
          "text": "Document with ID 1"
        }
      },
      {
        "_index": "test-index1",
        "_id": "2",
        "_score": 1,
        "_source": {
          "text": "Document with ID 2"
        }
      }
    ]
  }

Limitations of the _id field

While the _id field can be used in various queries, it is restricted from use in aggregations, sorting, and scripting. If you need to sort or aggregate on the _id field, it is recommended to duplicate the _id content into another field with doc_values enabled. Refer to IDs query for an example.