31_Metadata_source.asciidoc

Metadata: _source Field

By default, Elasticsearch stores the JSON string representing the document body in the _source field. Like all stored fields, the _source field is compressed before being written to disk.

This is almost always desired functionality because it means the following:

• The full document is available directly from the search results—​no need for a separate round-trip to fetch the document from another data store.

• Partial update requests will not function without the _source field.

• When your mapping changes and you need to reindex your data, you can do so directly from Elasticsearch instead of having to retrieve all of your documents from another (usually slower) data store.

• Individual fields can be extracted from the _source field and returned in get or search requests when you don’t need to see the whole document.

• It is easier to debug queries, because you can see exactly what each document contains, rather than having to guess their contents from a list of IDs.

That said, storing the _source field does use disk space. If none of the preceding reasons is important to you, you can disable the _source field with the following mapping:

PUT /my_index
{
    "mappings": {
        "my_type": {
            "_source": {
                "enabled":  false
            }
        }
    }
}

In a search request, you can ask for only certain fields by specifying the _source parameter in the request body:

GET /_search
{
    "query":   { "match_all": {}},
    "_source": [ "title", "created" ]
}

Values for these fields will be extracted from the _source field and returned instead of the full _source.

Stored Fields

Besides indexing the values of a field, you can also choose to store the original field value for later retrieval. Users with a Lucene background use stored fields to choose which fields they would like to be able to return in their search results. In fact, the _source field is a stored field.

In Elasticsearch, setting individual document fields to be stored is usually a false optimization. The whole document is already stored as the _source field. It is almost always better to just extract the fields that you need by using the _source parameter.