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_id field

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`_id` field

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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.

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).

# 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" ] 
    }
  }
}

Querying on the _id field (also see the 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.

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