OpenSearch/docs/reference/modules/scripting/fields.asciidoc

[[modules-scripting-fields]]
=== Accessing document fields and special variables

Depending on where a script is used, it will have access to certain special
variables and document fields.

[float]
== Update scripts

A script used in the <<docs-update,update>>,
<<docs-update-by-query,update-by-query>>, or <<docs-reindex,reindex>>
API will have access to the `ctx` variable which exposes:

[horizontal]
`ctx._source`::     Access to the document <<mapping-source-field,`_source` field>>.
`ctx.op`::          The operation that should be applied to the document: `index` or `delete`.
`ctx._index` etc::  Access to <<mapping-fields,document meta-fields>>, some of which may be read-only.

[float]
== Search and Aggregation scripts

With the exception of <<search-request-script-fields,script fields>> which are
executed once per search hit, scripts used in search and aggregations will be
executed once for every document which might match a query or an aggregation.
Depending on how many documents you have, this could mean millions or billions
of executions: these scripts need to be fast!

Field values can be accessed from a script using
<<modules-scripting-doc-vals,doc-values>>, or
<<modules-scripting-stored,stored fields or `_source` field>>, which are explained below.

[[scripting-score]]
[float]
=== Accessing the score of a document within a script

Scripts used in the <<query-dsl-function-score-query,`function_score` query>>,
in <<search-request-sort,script-based sorting>>, or in
<<search-aggregations,aggregations>> have access to the `_score` variable which
represents the current relevance score of a document.

Here's an example of using a script in a
<<query-dsl-function-score-query,`function_score` query>> to alter the
relevance `_score` of each document:

[source,js]
-------------------------------------
PUT my_index/_doc/1?refresh
{
  "text": "quick brown fox",
  "popularity": 1
}

PUT my_index/_doc/2?refresh
{
  "text": "quick fox",
  "popularity": 5
}

GET my_index/_search
{
  "query": {
    "function_score": {
      "query": {
        "match": {
          "text": "quick brown fox"
        }
      },
      "script_score": {
        "script": {
          "lang": "expression",
          "source": "_score * doc['popularity']"
        }
      }
    }
  }
}
-------------------------------------
// CONSOLE


[float]
[[modules-scripting-doc-vals]]
=== Doc Values

By far the fastest most efficient way to access a field value from a
script is to use the `doc['field_name']` syntax, which retrieves the field
value from <<doc-values,doc values>>. Doc values are a columnar field value
store, enabled by default on all fields except for <<text,analyzed `text` fields>>.

[source,js]
-------------------------------
PUT my_index/_doc/1?refresh
{
  "cost_price": 100
}

GET my_index/_search
{
  "script_fields": {
    "sales_price": {
      "script": {
        "lang":   "expression",
        "source": "doc['cost_price'] * markup",
        "params": {
          "markup": 0.2
        }
      }
    }
  }
}
-------------------------------
// CONSOLE

Doc-values can only return "simple" field values like numbers, dates, geo-
points, terms, etc, or arrays of these values if the field is multi-valued.
It cannot return JSON objects.

[NOTE]
.Missing fields
===================================================

The `doc['field']` will throw an error if `field` is missing from the mappings.
In `painless`, a check can first be done with `doc.containsKey('field')` to guard
accessing the `doc` map.  Unfortunately, there is no way to check for the
existence of the field in mappings in an `expression` script.

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

[NOTE]
.Doc values and `text` fields
===================================================

The `doc['field']` syntax can also be used for <<text,analyzed `text` fields>>
if <<fielddata,`fielddata`>> is enabled, but *BEWARE*: enabling fielddata on a
`text` field requires loading all of the terms into the JVM heap, which can be
very expensive both in terms of memory and CPU.  It seldom makes sense to
access `text` fields from scripts.

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

[float]
[[modules-scripting-stored]]
=== Stored Fields and `_source`

_Stored fields_ -- fields explicitly marked as
<<mapping-store,`"store": true`>> -- can be accessed using the
`_fields['field_name'].value` or `_fields['field_name']` syntax.

The document <<mapping-source-field,`_source`>>, which is really just a
special stored field,  can be accessed using the `_source.field_name` syntax.
The `_source` is loaded as a map-of-maps, so properties within object fields
can be accessed as, for example, `_source.name.first`.

[IMPORTANT]
.Prefer doc-values to stored fields
=========================================================

Stored fields (which includes the stored `_source` field) are much slower than
doc-values.  They are  optimised for returning several fields per result,
while doc values are optimised for accessing the value of a specific field in
many documents.


It makes sense to use `_source` or stored fields when generating a
<<search-request-script-fields,script field>> for the top ten hits from a search
result but, for other search and aggregation use cases, always prefer using
doc values.
=========================================================


For instance:

[source,js]
-------------------------------
PUT my_index?include_type_name=true
{
  "mappings": {
    "_doc": {
      "properties": {
        "title": { <1>
          "type": "text"
        },
        "first_name": {
          "type": "text",
          "store": true
        },
        "last_name": {
          "type": "text",
          "store": true
        }
      }
    }
  }
}

PUT my_index/_doc/1?refresh
{
  "title": "Mr",
  "first_name": "Barry",
  "last_name": "White"
}

GET my_index/_search
{
  "script_fields": {
    "source": {
      "script": {
        "lang": "painless",
        "source": "params._source.title + ' ' + params._source.first_name + ' ' + params._source.last_name" <2>
      }
    },
    "stored_fields": {
      "script": {
        "lang": "painless",
        "source": "params._fields['first_name'].value + ' ' + params._fields['last_name'].value"
      }
    }
  }
}
-------------------------------
// CONSOLE
<1> The `title` field is not stored and so cannot be used with the `_fields[]` syntax.
<2> The `title` field can still be accessed from the `_source`.

[TIP]
.Stored vs `_source`
=======================================================

The `_source` field is just a special stored field, so the performance is
similar to that of other stored fields.  The `_source` provides access to the
original document body that was indexed (including the ability to distinguish
`null` values from empty fields, single-value arrays from plain scalars, etc).

The only time it really makes sense to use stored fields instead of the
`_source` field is when the `_source` is very large and it is less costly to
access a few small stored fields instead of the entire `_source`.

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