elasticsearch/docs/reference/search/request/script-fields.asciidoc

[[search-request-script-fields]]
=== Script Fields

Allows to return a <<modules-scripting,script
evaluation>> (based on different fields) for each hit, for example:

[source,js]
--------------------------------------------------
{
    "query" : {
        ...
    },
    "script_fields" : {
        "test1" : {
            "script" : "doc['my_field_name'].value * 2"
        },
        "test2" : {
            "script" : "doc['my_field_name'].value * factor",
            "params" : {
                "factor"  : 2.0
            }
        }
    }
}
--------------------------------------------------

Script fields can work on fields that are not stored (`my_field_name` 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 indexed and
extract specific elements to be returned from it (can be an "object"
type). Here is an example:

[source,js]
--------------------------------------------------
    {
        "query" : {
            ...
        },
        "script_fields" : {
            "test1" : {
                "script" : "_source.obj1.obj2"
            }
        }
    }
--------------------------------------------------

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

It's important to understand the difference between
`doc['my_field'].value` and `_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 (can't return a json object from it) and make sense only on
non-analyzed or single term based fields.

The `_source` on the other hand causes the source to be loaded, parsed,
and then only the relevant part of the json is returned.