[[mapping-boost-field]]
=== `_boost`

deprecated[1.0.0.RC1,See <<function-score-instead-of-boost>>]

Boosting is the process of enhancing the relevancy of a document or
field. Field level mapping allows to define an explicit boost level on a
specific field. The boost field mapping (applied on the
<<mapping-root-object-type,root object>>) allows
to define a boost field mapping where *its content will control the
boost level of the document*. For example, consider the following
mapping:

[source,js]
--------------------------------------------------
{
    "tweet" : {
        "_boost" : {"name" : "my_boost", "null_value" : 1.0}
    }
}
--------------------------------------------------

The above mapping defines a mapping for a field named `my_boost`. If the
`my_boost` field exists within the JSON document indexed, its value will
control the boost level of the document indexed. For example, the
following JSON document will be indexed with a boost value of `2.2`:

[source,js]
--------------------------------------------------
{
    "my_boost" : 2.2,
    "message" : "This is a tweet!"
}
--------------------------------------------------

[[function-score-instead-of-boost]]
==== Function score instead of boost

Support for document boosting via the `_boost` field has been removed
from Lucene and is deprecated in Elasticsearch as of v1.0.0.RC1. The
implementation in Lucene resulted in unpredictable result when
used with multiple fields or multi-value fields.

Instead, the <<query-dsl-function-score-query>> can be used to achieve
the desired functionality by boosting each document by the value in
any field the document:

[source,js]
--------------------------------------------------
{
    "query": {
        "function_score": {
            "query": {  <1>
                "match": {
                    "title": "your main query"
                }
            },
            "functions": [{
                "field_value_factor": { <2>
                    "field": "my_boost_field"
                }
            }],
            "score_mode": "multiply"
        }
    }
}
--------------------------------------------------
<1> The original query, now wrapped in a `function_score` query.
<2> This function returns the value in `my_boost_field`, which is then
    multiplied by the query `_score` for each document.

Note, that `field_value_factor` is a 1.2.x feature.
