33_Metadata_ID.asciidoc

Metadata: Document Identity

There are four metadata fields associated with document identity:

_id

The string ID of the document

_type

The type name of the document

_index

The index where the document lives

_uid

The _type and _id concatenated together as type#id

By default, the _uid field is stored (can be retrieved) and indexed (searchable). The _type field is indexed but not stored, and the _id and _index fields are neither indexed nor stored, meaning they don’t really exist.

In spite of this, you can query the _id field as though it were a real field. Elasticsearch uses the _uid field to derive the _id. Although you can change the index and store settings for these fields, you almost never need to do so.

The _id field does have one setting that you may want to use: the path setting tells Elasticsearch that it should extract the value for the _id from a field within the document itself.

PUT /my_index
{
    "mappings": {
        "my_type": {
            "_id": {
                "path": "doc_id" (1)
            },
            "properties": {
                "doc_id": {
                    "type":   "string",
                    "index":  "not_analyzed"
                }
            }
        }
    }
}

1. Extract the doc _id from the doc_id field.

Then, when you index a document:

POST /my_index/my_type
{
    "doc_id": "123"
}

the _id value will be extracted from the doc_id field in the document body:

{
    "_index":   "my_index",
    "_type":    "my_type",
    "_id":      "123", (1)
    "_version": 1,
    "created":  true
}

1. The _id has been extracted correctly.

Warning

While this is very convenient, be aware that it has a slight performance impact on bulk requests (see [bulk-format]). The node handling the request can no longer use the optimized bulk format to parse just the metadata line in order to decide which shard should receive the request. Instead, it has to parse the document body as well.