histogram-aggregation.asciidoc

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 closes 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:

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:

{
    "aggs" : {
        "prices" : {
            "histogram" : {
                "field" : "price",
                "interval" : 50
            }
        }
    }
}

And the following may be the response:

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

The response above shows that non of the aggregated products has a price that falls within the range of [100 - 150). By default, the response will only contain the non-empty buckets, though it is possible to also return those, by setting the empty_buckets flag to true:

{
    "aggs" : {
        "prices" : {
            "histogram" : {
                "field" : "price",
                "interval" : 50,
                "empty_buckets" : true
            }
        }
    }
}

Response:

{
    "aggregations": {
        "prices": [
            {
                "key": 0,
                "doc_count": 2
            },
            {
                "key": 50,
                "doc_count": 4
            },
            {
                "key" : 100,
                "doc_count" : 0
            },
            {
                "key": 150,
                "doc_count": 3
            }
        ]
    }
}

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:

{
    "aggs" : {
        "prices" : {
            "histogram" : {
                "field" : "price",
                "interval" : 50,
                "order" : { "_key" : "desc" }
            }
        }
    }
}

Ordering the buckets by their doc_count - ascending:

{
    "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:

{
    "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.

{
    "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:

{
    "aggregations": {
        "prices": {
            "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 retuned as an ordered array. It is also possilbe to request the response as a hash instead keyed by the buckets keys:

{
    "aggs" : {
        "prices" : {
            "histogram" : {
                "field" : "price",
                "interval" : 50,
                "keyed" : true
            }
        }
    }
}

Response:

{
    "aggregations": {
        "prices": {
            "0": {
                "key": 0,
                "doc_count": 2
            },
            "50": {
                "key": 50,
                "doc_count": 4
            },
            "150": {
                "key": 150,
                "doc_count": 3
            }
        }
   }
}