[[search-aggregations-bucket-histogram-aggregation]]
=== Histogram

A multi-bucket values source based aggregation that can be applied on numeric values extracted from the documents.
It dynamically builds fixed size (a.k.a. interval) buckets over the values. For example, if the documents have a field
that holds a price (numeric), we can configure this aggregation to dynamically build buckets with interval `5`
(in case of price it may represent $5). When the aggregation executes, the price field of every document will be
evaluated and will be rounded down to its closest bucket - for example, if the price is `32` and the bucket size is `5`
then the rounding will yield `30` and thus the document will "fall" into the bucket that is associated withe the key `30`.
To make this more formal, here is the rounding function that is used:

[source,java]
--------------------------------------------------
rem = value % interval
if (rem < 0) {
    rem += interval
}
bucket_key = value - rem
--------------------------------------------------

The following snippet "buckets" the products based on their `price` by interval of `50`:

[source,js]
--------------------------------------------------
{
    "aggs" : {
        "prices" : {
            "histogram" : { 
                "field" : "price",
                "interval" : 50
            }
        }
    }
}
--------------------------------------------------

And the following may be the response:

[source,js]
--------------------------------------------------
{
    "aggregations": {
        "prices" : {
            "buckets": [
                {
                    "key": 0,
                    "doc_count": 2
                },
                {
                    "key": 50,
                    "doc_count": 4
                },
                {
                    "key": 150,
                    "doc_count": 3
                }
            ]
        }
    }
}
--------------------------------------------------

The response above shows that none of the aggregated products has a price that falls within the range of `[100 - 150)`.
By default, the response will only contain those buckets with a `doc_count` greater than 0. It is possible change that
and request buckets with either a higher minimum count or even 0 (in which case elasticsearch will "fill in the gaps"
and create buckets with zero documents). This can be configured using the `min_doc_count` setting:

[source,js]
--------------------------------------------------
{
    "aggs" : {
        "prices" : {
            "histogram" : { 
                "field" : "price",
                "interval" : 50,
                "min_doc_count" : 0
            }
        }
    }
}
--------------------------------------------------

Response:

[source,js]
--------------------------------------------------
{
    "aggregations": {
        "prices" : {
            "buckets": [
                {
                    "key": 0,
                    "doc_count": 2
                },
                {
                    "key": 50,
                    "doc_count": 4
                },
                {
                    "key" : 100,
                    "doc_count" : 0 <1>
                },
                {
                    "key": 150,
                    "doc_count": 3
                }
            ]
        }
    }
}
--------------------------------------------------

<1> No documents were found that belong in this bucket, yet it is still returned with zero `doc_count`.

==== Order

By default the returned buckets are sorted by their `key` ascending, though the order behaviour can be controled
using the `order` setting.

Ordering the buckets by their key - descending:

[source,js]
--------------------------------------------------
{
    "aggs" : {
        "prices" : {
            "histogram" : { 
                "field" : "price",
                "interval" : 50,
                "order" : { "_key" : "desc" }
            }
        }
    }
}
--------------------------------------------------

Ordering the buckets by their `doc_count` - ascending:

[source,js]
--------------------------------------------------
{
    "aggs" : {
        "prices" : {
            "histogram" : { 
                "field" : "price",
                "interval" : 50,
                "order" : { "_count" : "asc" }
            }
        }
    }
}
--------------------------------------------------

If the histogram aggregation has a direct metrics sub-aggregation, the latter can determine the order of the buckets:

[source,js]
--------------------------------------------------
{
    "aggs" : {
        "prices" : {
            "histogram" : { 
                "field" : "price",
                "interval" : 50,
                "order" : { "price_stats.min" : "asc" } <1>
            },
            "aggs" : {
                "price_stats" : { "stats" : {} } <2>
            }
        }
    }
}
--------------------------------------------------

<1> The `{ "price_stats.min" : asc" }` will sort the buckets based on `min` value of their their `price_stats` sub-aggregation.

<2> There is no need to configure the `price` field for the `price_stats` aggregation as it will inherit it by default from its parent histogram aggregation.

==== Minimum document count

It is possible to only return buckets that have a document count that is greater than or equal to a configured
limit through the `min_doc_count` option.

[source,js]
--------------------------------------------------
{
    "aggs" : {
        "prices" : {
            "histogram" : { 
                "field" : "price",
                "interval" : 50,
                "min_doc_count": 10
            }
        }
    }
}
--------------------------------------------------

The above aggregation would only return buckets that contain 10 documents or more. Default value is `1`.

NOTE:   The special value `0` can be used to add empty buckets to the response between the minimum and the maximum buckets.
Here is an example of what the response could look like:

[source,js]
--------------------------------------------------
{
    "aggregations": {
        "prices": {
            "buckets": {
                "0": {
                    "key": 0,
                    "doc_count": 2
                },
                "50": {
                    "key": 50,
                    "doc_count": 0
                },
                "150": {
                    "key": 150,
                    "doc_count": 3
                },
                "200": {
                    "key": 150,
                    "doc_count": 0
                },
                "250": {
                    "key": 150,
                    "doc_count": 0
                },
                "300": {
                    "key": 150,
                    "doc_count": 1
                }
            }
        }
   }
}
--------------------------------------------------

==== Response Format

By default, the buckets are returned as an ordered array. It is also possible to request the response as a hash
instead keyed by the buckets keys:

[source,js]
--------------------------------------------------
{
    "aggs" : {
        "prices" : {
            "histogram" : { 
                "field" : "price",
                "interval" : 50,
                "keyed" : true
            }
        }
    }
}
--------------------------------------------------

Response:

[source,js]
--------------------------------------------------
{
    "aggregations": {
        "prices": {
            "buckets": {
                "0": {
                    "key": 0,
                    "doc_count": 2
                },
                "50": {
                    "key": 50,
                    "doc_count": 4
                },
                "150": {
                    "key": 150,
                    "doc_count": 3
                }
            }
        }
    }
}
--------------------------------------------------