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

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

# Example documents
PUT my_index/_doc/1
{
  "text": "Document with ID 1"
}

PUT my_index/_doc/2&refresh=true
{
  "text": "Document with ID 2"
}

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

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.