[[search-aggregations-bucket-terms-aggregation]]
=== Terms

A multi-bucket value source based aggregation where buckets are dynamically built - one per unique value.

Example:

[source,js]
--------------------------------------------------
{
    "aggs" : {
        "genders" : {
            "terms" : { "field" : "gender" }
        }
    }
}
--------------------------------------------------

Response:

[source,js]
--------------------------------------------------
{
    ...

    "aggregations" : {
        "genders" : {
            "buckets" : [
                {
                    "key" : "male",
                    "doc_count" : 10
                },
                {
                    "key" : "female",
                    "doc_count" : 10
                },
            ]
        }
    }
}
--------------------------------------------------

By default, the `terms` aggregation will return the buckets for the top ten terms ordered by the `doc_count`. One can
change this default behaviour by setting the `size` parameter.

==== Size & Shard Size

The `size` parameter can be set to define how many term buckets should be returned out of the overall terms list. By
default, the node coordinating the search process will request each shard to provide its own top `size` term buckets
and once all shards respond, it will reduce the results to the final list that will then be returned to the client.
This means that if the number of unique terms is greater than `size`, the returned list is slightly off and not accurate
(it could be that the term counts are slightly off and it could even be that a term that should have been in the top
size buckets was not returned).

The higher the requested `size` is, the more accurate the results will be, but also, the more expensive it will be to
compute the final results (both due to bigger priority queues that are managed on a shard level and due to bigger data
transfers between the nodes and the client).

The `shard_size` parameter can be  used to minimize the extra work that comes with bigger requested `size`. When defined,
it will determine how many terms the coordinating node will request from each shard. Once all the shards responded, the
coordinating node will then reduce them to a final result which will be based on the `size` parameter - this way,
one can increase the accuracy of the returned terms and avoid the overhead of streaming a big list of buckets back to
the client.

NOTE:   `shard_size` cannot be smaller than `size` (as it doesn't make much sense). When it is, elasticsearch will
        override it and reset it to be equal to `size`.

added[1.1.0] It is possible to not limit the number of terms that are returned by setting `size` to `0`. Don't use this
on high-cardinality fields as this will kill both your CPU since terms need to be return sorted, and your network.

==== Order

The order of the buckets can be customized by setting the `order` parameter. By default, the buckets are ordered by
their `doc_count` descending. It is also possible to change this behaviour as follows:

Ordering the buckets by their `doc_count` in an ascending manner:

[source,js]
--------------------------------------------------
{
    "aggs" : {
        "genders" : {
            "terms" : { 
                "field" : "gender",
                "order" : { "_count" : "asc" }
            }
        }
    }
}
--------------------------------------------------

Ordering the buckets alphabetically by their terms in an ascending manner:

[source,js]
--------------------------------------------------
{
    "aggs" : {
        "genders" : {
            "terms" : { 
                "field" : "gender",
                "order" : { "_term" : "asc" }
            }
        }
    }
}
--------------------------------------------------


Ordering the buckets by single value metrics sub-aggregation (identified by the aggregation name):

[source,js]
--------------------------------------------------
{
    "aggs" : {
        "genders" : {
            "terms" : { 
                "field" : "gender",
                "order" : { "avg_height" : "desc" }
            },
            "aggs" : {
                "avg_height" : { "avg" : { "field" : "height" } }
            }
        }
    }
}
--------------------------------------------------

Ordering the buckets by multi value metrics sub-aggregation (identified by the aggregation name):

[source,js]
--------------------------------------------------
{
    "aggs" : {
        "genders" : {
            "terms" : { 
                "field" : "gender",
                "order" : { "height_stats.avg" : "desc" }
            },
            "aggs" : {
                "height_stats" : { "stats" : { "field" : "height" } }
            }
        }
    }
}
--------------------------------------------------

==== Minimum document count

It is possible to only return terms that match more than a configured number of hits using the `min_doc_count` option:

[source,js]
--------------------------------------------------
{
    "aggs" : {
        "tags" : {
            "terms" : { 
                "field" : "tag",
                "min_doc_count": 10
            }
        }
    }
}
--------------------------------------------------

The above aggregation would only return tags which have been found in 10 hits or more. Default value is `1`.

NOTE:    Setting `min_doc_count`=`0` will also return buckets for terms that didn't match any hit. However, some of
         the returned terms which have a document count of zero might only belong to deleted documents, so there is
         no warranty that a `match_all` query would find a positive document count for those terms.

WARNING: When NOT sorting on `doc_count` descending, high values of `min_doc_count` may return a number of buckets
         which is less than `size` because not enough data was gathered from the shards. Missing buckets can be
         back by increasing `shard_size`.

==== Script

Generating the terms using a script:

[source,js]
--------------------------------------------------
{
    "aggs" : {
        "genders" : {
            "terms" : { 
                "script" : "doc['gender'].value"
            }
        }
    }
}
--------------------------------------------------

==== Value Script

[source,js]
--------------------------------------------------
{
    "aggs" : {
        "genders" : {
            "terms" : { 
                "field" : "gender",
                "script" : "'Gender: ' +_value"
            }
        }
    }
}
--------------------------------------------------


==== Filtering Values

It is possible to filter the values for which buckets will be created. This can be done using the `include` and
`exclude` parameters which are based on regular expressions.

[source,js]
--------------------------------------------------
{
    "aggs" : {
        "tags" : {
            "terms" : {
                "field" : "tags",
                "include" : ".*sport.*",
                "exclude" : "water_.*"
            }
        }
    }
}
--------------------------------------------------

In the above example, buckets will be created for all the tags that has the word `sport` in them, except those starting
with `water_` (so the tag `water_sports` will no be aggregated). The `include` regular expression will determine what
values are "allowed" to be aggregated, while the `exclude` determines the values that should not be aggregated. When
both are defined, the `exclude` has precedence, meaning, the `include` is evaluated first and only then the `exclude`.

The regular expression are based on the Java(TM) http://docs.oracle.com/javase/7/docs/api/java/util/regex/Pattern.html[Pattern],
and as such, they it is also possible to pass in flags that will determine how the compiled regular expression will work:

[source,js]
--------------------------------------------------
{
    "aggs" : {
        "tags" : {
             "terms" : {
                 "field" : "tags",
                 "include" : {
                     "pattern" : ".*sport.*",
                     "flags" : "CANON_EQ|CASE_INSENSITIVE" <1>
                 },
                 "exclude" : {
                     "pattern" : "water_.*",
                     "flags" : "CANON_EQ|CASE_INSENSITIVE"
                 }
             }
         }
    }
}
--------------------------------------------------

<1> the flags are concatenated using the `|` character as a separator

The possible flags that can be used are:
http://docs.oracle.com/javase/7/docs/api/java/util/regex/Pattern.html#CANON_EQ[`CANON_EQ`],
http://docs.oracle.com/javase/7/docs/api/java/util/regex/Pattern.html#CASE_INSENSITIVE[`CASE_INSENSITIVE`],
http://docs.oracle.com/javase/7/docs/api/java/util/regex/Pattern.html#COMMENTS[`COMMENTS`],
http://docs.oracle.com/javase/7/docs/api/java/util/regex/Pattern.html#DOTALL[`DOTALL`],
http://docs.oracle.com/javase/7/docs/api/java/util/regex/Pattern.html#LITERAL[`LITERAL`],
http://docs.oracle.com/javase/7/docs/api/java/util/regex/Pattern.html#MULTILINE[`MULTILINE`],
http://docs.oracle.com/javase/7/docs/api/java/util/regex/Pattern.html#UNICODE_CASE[`UNICODE_CASE`],
http://docs.oracle.com/javase/7/docs/api/java/util/regex/Pattern.html#UNICODE_CHARACTER_CLASS[`UNICODE_CHARACTER_CLASS`] and
http://docs.oracle.com/javase/7/docs/api/java/util/regex/Pattern.html#UNIX_LINES[`UNIX_LINES`]

==== Execution hint

There are two mechanisms by which terms aggregations can be executed: either by using field values directly in order to aggregate
data per-bucket (`map`), or by using ordinals of the field values instead of the values themselves (`ordinals`). Although the
latter execution mode can be expected to be slightly faster, it is only available for use when the underlying data source exposes
those terms ordinals. Moreover, it may actually be slower if most field values are unique. Elasticsearch tries to have sensible
defaults when it comes to the execution mode that should be used, but in case you know that one execution mode may perform better
than the other one, you have the ability to "hint" it to Elasticsearch:

[source,js]
--------------------------------------------------
{
    "aggs" : {
        "tags" : {
             "terms" : {
                 "field" : "tags",
                 "execution_hint": "map" <1>
             }
         }
    }
}
--------------------------------------------------

<1> the possible values are `map` and `ordinals`

Please note that Elasticsearch will ignore this execution hint if it is not applicable.
