druid/docs/content/Post-aggregations.md

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Post-aggregations are specifications of processing that should happen on aggregated values as they come out of Druid. If you include a post aggregation as part of a query, make sure to include all aggregators the post-aggregator requires.

There are several post-aggregators available.

Arithmetic post-aggregator

The arithmetic post-aggregator applies the provided function to the given fields from left to right. The fields can be aggregators or other post aggregators.

Supported functions are +, -, *, and /

The grammar for an arithmetic post aggregation is:

<code>postAggregation : {
    "type"  : "arithmetic",
    "name"  : <output_name>,
    "fn"    : <arithmetic_function>,
    "fields": [<post_aggregator>, <post_aggregator>, ...]
}</code>

Field accessor post-aggregator

This returns the value produced by the specified aggregator|Aggregations.

fieldName refers to the output name of the aggregator given in the aggregations|Aggregations portion of the query.

<code>field_accessor : {
    "type"      : "fieldAccess",
    "fieldName" : <aggregator_name>
}</code>

Constant post-aggregator

The constant post-aggregator always returns the specified value.

<code>constant : {
    "type"  : "constant",
    "name"  : <output_name>,
    "value" : <numerical_value>,
}</code>

Example Usage

In this example, let’s calculate a simple percentage using post aggregators. Let’s imagine our data set has a metric called “total”.

The format of the query JSON is as follows:

<code>
{
    ...
    "aggregations" : [
        {
            "type" : "count",
            "name" : "rows"
        },
        {
            "type"      : "doubleSum",
            "name"      : "tot",
            "fieldName" : "total"
        }
    ],
    "postAggregations" : {
        "type"   : "arithmetic",
        "name"   : "average",
        "fn"     : "*",
        "fields" : [
            {
                "type"   : "arithmetic",
                "name"   : "div",
                "fn"     : "/",
                "fields" : [
                    {
                        "type"      : "fieldAccess",
                        "name"      : "tot",
                        "fieldName" : "tot"
                     },
                     {
                        "type"      : "fieldAccess",
                        "name"      : "rows",
                        "fieldName" : "rows"
                     }
                ]
            },
            {
                "type" : "constant",
                "name": "const",
                "value" : 100
            }
        ]
    }
    ...
}
</code>