[[search-aggregations-pipeline-bucket-sort-aggregation]]
=== Bucket Sort Aggregation

A parent pipeline aggregation which sorts the buckets of its parent multi-bucket aggregation.
Zero or more sort fields may be specified together with the corresponding sort order.
Each bucket may be sorted based on its `_key`, `_count` or its sub-aggregations.
In addition, parameters `from` and `size` may be set in order to truncate the result buckets.

NOTE: The `bucket_sort` aggregation, like all pipeline aggregations, is executed after all other non-pipeline aggregations.
This means the sorting only applies to whatever buckets are already returned from the parent aggregation. For example,
if the parent aggregation is `terms` and its `size` is set to `10`, the `bucket_sort` will only sort over those 10
returned term buckets.

==== Syntax

A `bucket_sort` aggregation looks like this in isolation:

[source,js]
--------------------------------------------------
{
  "bucket_sort": {
    "sort": [
      { "sort_field_1": { "order": "asc" } },   <1>
      { "sort_field_2": { "order": "desc" } },
      "sort_field_3"
    ],
    "from": 1,
    "size": 3
  }
}
--------------------------------------------------
// NOTCONSOLE
<1> Here, `sort_field_1` is the bucket path to the variable to be used as the primary sort and its order
is ascending.

[[bucket-sort-params]]
.`bucket_sort` Parameters
[options="header"]
|===
|Parameter Name |Description |Required |Default Value
|`sort` |The list of fields to sort on. See <<sort-search-results,`sort`>> for more details. |Optional |
|`from` |Buckets in positions prior to the set value will be truncated. |Optional | `0`
|`size` |The number of buckets to return. Defaults to all buckets of the parent aggregation. |Optional |
|`gap_policy` |The policy to apply when gaps are found in the data (see <<gap-policy>> for more
 details)|Optional |`skip`
|===

The following snippet returns the buckets corresponding to the 3 months with the highest total sales in descending order:

[source,console]
--------------------------------------------------
POST /sales/_search
{
  "size": 0,
  "aggs": {
    "sales_per_month": {
      "date_histogram": {
        "field": "date",
        "calendar_interval": "month"
      },
      "aggs": {
        "total_sales": {
          "sum": {
            "field": "price"
          }
        },
        "sales_bucket_sort": {
          "bucket_sort": {
            "sort": [
              { "total_sales": { "order": "desc" } } <1>
            ],
            "size": 3                                <2>
          }
        }
      }
    }
  }
}
--------------------------------------------------
// TEST[setup:sales]

<1> `sort` is set to use the values of `total_sales` in descending order
<2> `size` is set to `3` meaning only the top 3 months in `total_sales` will be returned

And the following may be the response:

[source,console-result]
--------------------------------------------------
{
   "took": 82,
   "timed_out": false,
   "_shards": ...,
   "hits": ...,
   "aggregations": {
      "sales_per_month": {
         "buckets": [
            {
               "key_as_string": "2015/01/01 00:00:00",
               "key": 1420070400000,
               "doc_count": 3,
               "total_sales": {
                   "value": 550.0
               }
            },
            {
               "key_as_string": "2015/03/01 00:00:00",
               "key": 1425168000000,
               "doc_count": 2,
               "total_sales": {
                   "value": 375.0
               },
            },
            {
               "key_as_string": "2015/02/01 00:00:00",
               "key": 1422748800000,
               "doc_count": 2,
               "total_sales": {
                   "value": 60.0
               },
            }
         ]
      }
   }
}
--------------------------------------------------
// TESTRESPONSE[s/"took": 82/"took": $body.took/]
// TESTRESPONSE[s/"_shards": \.\.\./"_shards": $body._shards/]
// TESTRESPONSE[s/"hits": \.\.\./"hits": $body.hits/]

==== Truncating without sorting

It is also possible to use this aggregation in order to truncate the result buckets
without doing any sorting. To do so, just use the `from` and/or `size` parameters
without specifying `sort`.

The following example simply truncates the result so that only the second bucket is returned:

[source,console]
--------------------------------------------------
POST /sales/_search
{
  "size": 0,
  "aggs": {
    "sales_per_month": {
      "date_histogram": {
        "field": "date",
        "calendar_interval": "month"
      },
      "aggs": {
        "bucket_truncate": {
          "bucket_sort": {
            "from": 1,
            "size": 1
          }
        }
      }
    }
  }
}
--------------------------------------------------
// TEST[setup:sales]

Response:

[source,console-result]
--------------------------------------------------
{
   "took": 11,
   "timed_out": false,
   "_shards": ...,
   "hits": ...,
   "aggregations": {
      "sales_per_month": {
         "buckets": [
            {
               "key_as_string": "2015/02/01 00:00:00",
               "key": 1422748800000,
               "doc_count": 2
            }
         ]
      }
   }
}
--------------------------------------------------
// TESTRESPONSE[s/"took": 11/"took": $body.took/]
// TESTRESPONSE[s/"_shards": \.\.\./"_shards": $body._shards/]
// TESTRESPONSE[s/"hits": \.\.\./"hits": $body.hits/]
