OpenSearch/docs/reference/search/search-your-data/retrieve-selected-fields.asciidoc

[[search-fields]]
== Retrieve selected fields from a search
++++
<titleabbrev>Retrieve selected fields</titleabbrev>
++++

By default, each hit in the search response includes the document
<<mapping-source-field,`_source`>>, which is the entire JSON object that was
provided when indexing the document. To retrieve specific fields in the search
response, you can use the `fields` parameter:

[source,console]
----
POST my-index-000001/_search
{
  "query": {
    "match": {
      "message": "foo"
    }
  },
  "fields": ["user.id", "@timestamp"],
  "_source": false
}
----
// TEST[setup:my_index]

The `fields` parameter consults both a document's `_source` and the index
mappings to load and return values. Because it makes use of the mappings,
`fields` has some advantages over referencing the `_source` directly: it
accepts <<multi-fields, multi-fields>> and <<alias, field aliases>>, and
also formats field values like dates in a consistent way.

A document's `_source` is stored as a single field in Lucene. So the whole
`_source` object must be loaded and parsed even if only a small number of
fields are requested. To avoid this limitation, you can try another option for
loading fields:

* Use the <<docvalue-fields, `docvalue_fields`>>
parameter to get values for selected fields. This can be a good
choice when returning a fairly small number of fields that support doc values,
such as keywords and dates.
* Use the <<request-body-search-stored-fields, `stored_fields`>> parameter to
get the values for specific stored fields (fields that use the
<<mapping-store,`store`>> mapping option).

If needed, you can use the <<script-fields,`script_field`>> parameter to
transform field values in the response using a script. However, scripts can’t
make use of {es}'s index structures or related optimizations. This can sometimes
result in slower search speeds.

You can find more detailed information on each of these methods in the
following sections:

* <<search-fields-param>>
* <<docvalue-fields>>
* <<stored-fields>>
* <<source-filtering>>
* <<script-fields>>

[discrete]
[[search-fields-param]]
=== Fields

The `fields` parameter allows for retrieving a list of document fields in
the search response. It consults both the document `_source` and the index
mappings to return each value in a standardized way that matches its mapping
type. By default, date fields are formatted according to the
<<mapping-date-format,date format>> parameter in their mappings.

The following search request uses the `fields` parameter to retrieve values
for the `user.id` field, all fields starting with `http.response.`, and the
`@timestamp` field:

[source,console]
----
POST my-index-000001/_search
{
  "query": {
    "match": {
      "user.id": "kimchy"
    }
  },
  "fields": [
    "user.id",
    "http.response.*", <1>
    {
      "field": "@timestamp",
      "format": "epoch_millis" <2>
    }
  ],
  "_source": false
}
----
// TEST[setup:my_index]

<1> Both full field names and wildcard patterns are accepted.
<2> Using object notation, you can pass a `format` parameter to apply a custom
    format for the field's values. The date fields
    <<date,`date`>> and <<date_nanos, `date_nanos`>> accept a
    <<mapping-date-format,date format>>. <<spatial_datatypes, Spatial fields>>
    accept either `geojson` for http://www.geojson.org[GeoJSON] (the default)
    or `wkt` for
    {wikipedia}/Well-known_text_representation_of_geometry[Well Known Text].
    Other field types do not support the `format` parameter.

The values are returned as a flat list in the `fields` section in each hit:

[source,console-result]
----
{
  "took" : 2,
  "timed_out" : false,
  "_shards" : {
    "total" : 1,
    "successful" : 1,
    "skipped" : 0,
    "failed" : 0
  },
  "hits" : {
    "total" : {
      "value" : 1,
      "relation" : "eq"
    },
    "max_score" : 1.0,
    "hits" : [
      {
        "_index" : "my-index-000001",
        "_id" : "0",
        "_score" : 1.0,
        "_type" : "_doc",
        "fields" : {
          "user.id" : [
            "kimchy"
          ],
          "@timestamp" : [
            "4098435132000"
          ],
          "http.response.bytes": [
            1070000
          ],
          "http.response.status_code": [
            200
          ]
        }
      }
    ]
  }
}
----
// TESTRESPONSE[s/"took" : 2/"took": $body.took/]
// TESTRESPONSE[s/"max_score" : 1.0/"max_score" : $body.hits.max_score/]
// TESTRESPONSE[s/"_score" : 1.0/"_score" : $body.hits.hits.0._score/]

Only leaf fields are returned -- `fields` does not allow for fetching entire
objects.

The `fields` parameter handles field types like <<alias, field aliases>> and
<<constant-keyword, `constant_keyword`>> whose values aren't always present in
the `_source`. Other mapping options are also respected, including
<<ignore-above, `ignore_above`>>, <<ignore-malformed, `ignore_malformed`>> and
<<null-value, `null_value`>>.

NOTE: The `fields` response always returns an array of values for each field,
even when there is a single value in the `_source`. This is because {es} has
no dedicated array type, and any field could contain multiple values. The
`fields` parameter also does not guarantee that array values are returned in
a specific order. See the mapping documentation on <<array, arrays>> for more
background.



[discrete]
[[docvalue-fields]]
=== Doc value fields

You can use the <<docvalue-fields,`docvalue_fields`>> parameter to return
<<doc-values,doc values>> for one or more fields in the search response.

Doc values store the same values as the `_source` but in an on-disk,
column-based structure that's optimized for sorting and aggregations. Since each
field is stored separately, {es} only reads the field values that were requested
and can avoid loading the whole document `_source`.

Doc values are stored for supported fields by default. However, doc values are
not supported for <<text,`text`>> or
{plugins}/mapper-annotated-text-usage.html[`text_annotated`] fields.

The following search request uses the `docvalue_fields` parameter to retrieve
doc values for the `user.id` field, all fields starting with `http.response.`, and the
`@timestamp` field:

[source,console]
----
GET my-index-000001/_search
{
  "query": {
    "match": {
      "user.id": "kimchy"
    }
  },
  "docvalue_fields": [
    "user.id",
    "http.response.*", <1>
    {
      "field": "date",
      "format": "epoch_millis" <2>
    }
  ]
}
----
// TEST[setup:my_index]

<1> Both full field names and wildcard patterns are accepted.
<2> Using object notation, you can pass a `format` parameter to apply a custom
    format for the field's doc values. <<date,Date fields>> support a
    <<mapping-date-format,date `format`>>. <<number,Numeric fields>> support a
    https://docs.oracle.com/javase/8/docs/api/java/text/DecimalFormat.html[DecimalFormat
    pattern]. Other field datatypes do not support the `format` parameter.

TIP: You cannot use the `docvalue_fields` parameter to retrieve doc values for
nested objects. If you specify a nested object, the search returns an empty
array (`[ ]`) for the field. To access nested fields, use the
<<inner-hits, `inner_hits`>> parameter's `docvalue_fields`
property.

[discrete]
[[stored-fields]]
=== Stored fields

It's also possible to store an individual field's values by using the
<<mapping-store,`store`>> mapping option. You can use the
`stored_fields` parameter to include these stored values in the search response.

WARNING: The `stored_fields` parameter is for fields that are explicitly marked as
stored in the mapping, which is off by default and generally not recommended.
Use <<source-filtering,source filtering>> instead to select
subsets of the original source document to be returned.

Allows to selectively load specific stored fields for each document represented
by a search hit.

[source,console]
--------------------------------------------------
GET /_search
{
  "stored_fields" : ["user", "postDate"],
  "query" : {
    "term" : { "user" : "kimchy" }
  }
}
--------------------------------------------------

`*` can be used to load all stored fields from the document.

An empty array will cause only the `_id` and `_type` for each hit to be
returned, for example:

[source,console]
--------------------------------------------------
GET /_search
{
  "stored_fields" : [],
  "query" : {
    "term" : { "user" : "kimchy" }
  }
}
--------------------------------------------------

If the requested fields are not stored (`store` mapping set to `false`), they will be ignored.

Stored field values fetched from the document itself are always returned as an array. On the contrary, metadata fields like `_routing` are never returned as an array.

Also only leaf fields can be returned via the `stored_fields` option. If an object field is specified, it will be ignored.

NOTE: On its own, `stored_fields` cannot be used to load fields in nested
objects -- if a field contains a nested object in its path, then no data will
be returned for that stored field. To access nested fields, `stored_fields`
must be used within an <<inner-hits, `inner_hits`>> block.

[discrete]
[[disable-stored-fields]]
==== Disable stored fields

To disable the stored fields (and metadata fields) entirely use: `_none_`:

[source,console]
--------------------------------------------------
GET /_search
{
  "stored_fields": "_none_",
  "query" : {
    "term" : { "user" : "kimchy" }
  }
}
--------------------------------------------------

NOTE: <<source-filtering,`_source`>> and <<request-body-search-version, `version`>> parameters cannot be activated if `_none_` is used.

[discrete]
[[source-filtering]]
=== Source filtering

You can use the `_source` parameter to select what fields of the source are
returned. This is called _source filtering_.

The following search API request sets the `_source` request body parameter to
`false`. The document source is not included in the response.

[source,console]
----
GET /_search
{
  "_source": false,
  "query": {
    "match": {
      "user.id": "kimchy"
    }
  }
}
----

To return only a subset of source fields, specify a wildcard (`*`) pattern in
the `_source` parameter. The following search API request returns the source for
only the `obj` field and its properties.

[source,console]
----
GET /_search
{
  "_source": "obj.*",
  "query": {
    "match": {
      "user.id": "kimchy"
    }
  }
}
----

You can also specify an array of wildcard patterns in the `_source` field. The
following search API request returns the source for only the `obj1` and
`obj2` fields and their properties.

[source,console]
----
GET /_search
{
  "_source": [ "obj1.*", "obj2.*" ],
  "query": {
    "match": {
      "user.id": "kimchy"
    }
  }
}
----

For finer control, you can specify an object containing arrays of `includes` and
`excludes` patterns in the `_source` parameter.

If the `includes` property is specified, only source fields that match one of
its patterns are returned. You can exclude fields from this subset using the
`excludes` property.

If the `includes` property is not specified, the entire document source is
returned, excluding any fields that match a pattern in the `excludes` property.

The following search API request returns the source for only the `obj1` and
`obj2` fields and their properties, excluding any child `description` fields.

[source,console]
----
GET /_search
{
  "_source": {
    "includes": [ "obj1.*", "obj2.*" ],
    "excludes": [ "*.description" ]
  },
  "query": {
    "term": {
      "user.id": "kimchy"
    }
  }
}
----

[discrete]
[[script-fields]]
=== Script fields

You can use the `script_fields` parameter to retrieve a <<modules-scripting,script
evaluation>> (based on different fields) for each hit. For example:

[source,console]
--------------------------------------------------
GET /_search
{
  "query": {
    "match_all": {}
  },
  "script_fields": {
    "test1": {
      "script": {
        "lang": "painless",
        "source": "doc['price'].value * 2"
      }
    },
    "test2": {
      "script": {
        "lang": "painless",
        "source": "doc['price'].value * params.factor",
        "params": {
          "factor": 2.0
        }
      }
    }
  }
}
--------------------------------------------------
// TEST[setup:sales]

Script fields can work on fields that are not stored (`price` in
the above case), and allow to return custom values to be returned (the
evaluated value of the script).

Script fields can also access the actual `_source` document and
extract specific elements to be returned from it by using `params['_source']`.
Here is an example:

[source,console]
--------------------------------------------------
GET /_search
    {
        "query" : {
            "match_all": {}
        },
        "script_fields" : {
            "test1" : {
                "script" : "params['_source']['message']"
            }
        }
    }
--------------------------------------------------
// TEST[setup:my_index]

Note the `_source` keyword here to navigate the json-like model.

It's important to understand the difference between
`doc['my_field'].value` and `params['_source']['my_field']`. The first,
using the doc keyword, will cause the terms for that field to be loaded to
memory (cached), which will result in faster execution, but more memory
consumption. Also, the `doc[...]` notation only allows for simple valued
fields (you can't return a json object from it) and makes sense only for
non-analyzed or single term based fields. However, using `doc` is
still the recommended way to access values from the document, if at all
possible, because `_source` must be loaded and parsed every time it's used.
Using `_source` is very slow.