[[mapping-source-field]]
=== `_source`

The `_source` field is an automatically generated field that stores the actual
JSON that was used as the indexed document. It is not indexed (searchable),
just stored. When executing "fetch" requests, like <<docs-get,get>> or
<<search-search,search>>, the `_source` field is returned by default.

==== Disabling source

Though very handy to have around, the source field does incur storage overhead
within the index. For this reason, it can be disabled as follows:

[source,js]
--------------------------------------------------
PUT tweets
{
  "mappings": {},
  "tweet": {
    "_source": {
      "enabled": false
    }
  }
}
--------------------------------------------------
// AUTOSENSE

[WARNING]
.Think before disabling the source field
==================================================

Users often disable the `_source` field without thinking about the
consequences, and then live to regret it.  If the `_source` field isn't
available then a number of features are not supported:

* The <<docs-update,`update` API>>.

* On the fly <<search-request-highlighting,highlighting>>.

* The ability to reindex from one Elasticsearch index to another, either
  to change mappings or analysis, or to upgrade an index to a new major
  version.

* The ability to debug queries or aggregations by viewing the original
  document used at index time.

* Potentially in the future, the ability to repair index corruption
  automatically.

==================================================

.The metrics use case
**************************************************

The _metrics_ use case is distinct from other time-based or logging use cases
in that there are many small documents which consist only of numbers, dates,
or keywords.  There are no updates, no highlighting requests, and the data
ages quickly so there is no need to reindex.  Search requests typically use
simple queries to filter the dataset by date or tags, and the results are
returned as aggregations.

In this case, disabling the `_source` field will save space and reduce I/O.
It is also advisable to disable the <<mapping-all-field,`_all` field>> in the
metrics case.

**************************************************


[[include-exclude]]
==== Including / Excluding fields from source

An expert-only feature is the ability to prune the contents of the `_source`
field after the document has been indexed, but before the `_source` field is
stored.  The `includes`/`excludes` parameters (which also accept wildcards)
can be used as follows:

[source,js]
--------------------------------------------------
PUT logs
{
  "mappings": {
    "event": {
      "_source": {
        "includes": [
          "*.count",
          "meta.*"
        ],
        "excludes": [
          "meta.description",
          "meta.other.*"
        ]
      }
    }
  }
}

PUT logs/event/1
{
  "requests": {
    "count": 10,
    "foo": "bar" <1>
  },
  "meta": {
    "name": "Some metric",
    "description": "Some metric description", <1>
    "other": {
      "foo": "one", <1>
      "baz": "two" <1>
    }
  }
}

GET logs/event/_search
{
  "query": {
    "match": {
      "meta.other.foo": "one" <2>
    }
  }
}
--------------------------------------------------
// AUTOSENSE

<1> These fields will be removed from the stored `_source` field.
<2> We can still search on this field, even though it is not in the stored `_source`.

WARNING: Removing fields from the `_source` has similar downsides to disabling
`_source`, especially the fact that you cannot reindex documents from one
Elasticsearch index to another. Consider using
<<search-request-source-filtering,source filtering>> or a
<<mapping-transform,transform script>> instead.
