OpenSearch/docs/reference/aggregations/pipeline/bucket-selector-aggregation.asciidoc

[[search-aggregations-pipeline-bucket-selector-aggregation]]
=== Bucket Selector Aggregation

coming[2.0.0-beta1]

experimental[]

A parent pipeline aggregation which executes a script which determines whether the current bucket will be retained 
in the parent multi-bucket aggregation. The specified metric must be numeric and the script must return a boolean value. 
If the script language is `expression` then a numeric return value is permitted. In this case 0.0 will be evaluated as `false` 
and all other values will evaluate to true.

Note: The bucket_selector aggregation, like all pipeline aggregations, executions after all other sibling aggregations. This means that 
using the bucket_selector aggregation to filter the returned buckets in the response does not save on execution time running the aggregations.

==== Syntax

A `bucket_selector` aggregation looks like this in isolation:

[source,js]
--------------------------------------------------
{
    "bucket_selector": {
        "buckets_path": {
            "my_var1": "the_sum", <1>
            "my_var2": "the_value_count"
        },
        script: "my_var1 > my_var2"
    }
}
--------------------------------------------------
<1> Here, `my_var1` is the name of the variable for this buckets path to use in the script, `the_sum` is the path to 
the metrics to use for that variable.


.`bucket_selector` Parameters
|===
|Parameter Name |Description |Required |Default Value
|`script` |The script to run for this aggregation. The script can be inline, file or indexed. (see <<modules-scripting>> 
for more details) |Required |
|`buckets_path` |A map of script variables and their associated path to the buckets we wish to use for the variable 
(see <<bucket-path-syntax>> for more details) |Required |
 |`gap_policy` |The policy to apply when gaps are found in the data (see <<gap-policy>> for more
 details)|Optional, defaults to `skip` |
|===

The following snippet only retains buckets where the total sales for the month is less than or equal to 50:

[source,js]
--------------------------------------------------
{
    "aggs" : {
        "sales_per_month" : {
            "date_histogram" : {
                "field" : "date",
                "interval" : "month"
            },
            "aggs": {
                "total_sales": {
                    "sum": {
                        "field": "price"
                    }
                }
                "sales_bucket_filter": {
                    "bucket_selector": {
                        "buckets_paths": {
                          "totalSales": "total_sales"
                        },
                        "script": "totalSales <= 50"
                    }
                }
            }
        }
    }
}
--------------------------------------------------

And the following may be the response:

[source,js]
--------------------------------------------------
{
   "aggregations": {
      "sales_per_month": {
         "buckets": [
            {
               "key_as_string": "2015/01/01 00:00:00",
               "key": 1420070400000,
               "doc_count": 3,
               "total_sales": {
                   "value": 50
               }
            },<1>
            {
               "key_as_string": "2015/03/01 00:00:00",
               "key": 1425168000000,
               "doc_count": 2,
               "total_sales": {
                   "value": 40
               },
            }
         ]
      }
   }
}
--------------------------------------------------
<1> Bucket for `2015/02/01 00:00:00` has been removed as its total sales exceeded 50