[[search-explain]]
== Explain API

The explain api computes a score explanation for a query and a specific
document. This can give useful feedback whether a document matches or
didn't match a specific query.

The `index` and `type` parameters expect a single index and a single
type respectively.

[float]
=== Usage

Full query example:

[source,js]
--------------------------------------------------
GET /twitter/tweet/0/_explain
{
      "query" : {
        "match" : { "message" : "elasticsearch" }
      }
}
--------------------------------------------------
// CONSOLE
// TEST[setup:twitter]

This will yield the following result:

[source,js]
--------------------------------------------------
{
  "_index" : "twitter",
  "_type" : "tweet",
  "_id" : "0",
  "matched" : true,
  "explanation" : {
    "value" : 1.55077,
    "description" : "sum of:",
    "details" : [ {
      "value" : 1.55077,
      "description" : "weight(message:elasticsearch in 0) [PerFieldSimilarity], result of:",
      "details" : [ {
        "value" : 1.55077,
        "description" : "score(doc=0,freq=1.0 = termFreq=1.0\n), product of:",
        "details" : [ {
          "value" : 1.3862944,
          "description" : "idf(docFreq=1, docCount=5)",
          "details" : [ ]
        }, {
          "value" : 1.1186441,
          "description" : "tfNorm, computed from:",
          "details" : [
            { "value" : 1.0, "description" : "termFreq=1.0", "details" : [ ] },
            { "value" : 1.2, "description" : "parameter k1", "details" : [ ] },
            { "value" : 0.75, "description" : "parameter b", "details" : [ ] },
            { "value" : 5.4, "description" : "avgFieldLength", "details" : [ ] },
            { "value" : 4.0, "description" : "fieldLength", "details" : [ ] }
          ]
        } ]
      } ]
    }, {
      "value" : 0.0,
      "description" : "match on required clause, product of:",
      "details" : [ {
        "value" : 0.0,
        "description" : "# clause",
        "details" : [ ]
      }, {
        "value" : 1.0,
        "description" : "*:*, product of:",
        "details" : [
          { "value" : 1.0, "description" : "boost", "details" : [ ] },
          { "value" : 1.0, "description" : "queryNorm", "details" : [ ] }
        ]
      } ]
    } ]
  }
}
--------------------------------------------------
// TESTRESPONSE

There is also a simpler way of specifying the query via the `q`
parameter. The specified `q` parameter value is then parsed as if the
`query_string` query was used. Example usage of the `q` parameter in the
explain api:

[source,js]
--------------------------------------------------
GET /twitter/tweet/0/_explain?q=message:search
--------------------------------------------------
// CONSOLE
// TEST[setup:twitter]

This will yield the same result as the previous request.

[float]
=== All parameters:

[horizontal]
`_source`::

    Set to `true` to retrieve the `_source` of the document explained. You can also
    retrieve part of the document by using `_source_include` & `_source_exclude` (see <<get-source-filtering,Get API>> for more details)

`stored_fields`::
    Allows to control which stored fields to return as part of the
    document explained.

`routing`::
    Controls the routing in the case the routing was used
    during indexing.

`parent`::
    Same effect as setting the routing parameter.

`preference`::
    Controls on which shard the explain is executed.

`source`::
    Allows the data of the request to be put in the query
    string of the url.

`q`::
    The query string (maps to the query_string query).

`df`::
    The default field to use when no field prefix is defined within
    the query. Defaults to _all field.

`analyzer`::
    The analyzer name to be used when analyzing the query
    string. Defaults to the analyzer of the _all field.

`analyze_wildcard`::
    Should wildcard and prefix queries be analyzed or
    not. Defaults to false.

`lowercase_expanded_terms`::
    Should terms be automatically lowercased
    or not. Defaults to true.

`lenient`::
    If set to true will cause format based failures (like
    providing text to a numeric field) to be ignored. Defaults to false.

`default_operator`::
    The default operator to be used, can be AND or
    OR. Defaults to OR.