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.
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:
postAggregation : { "type" : "arithmetic", "name" : <output_name>, "fn" : <arithmetic_function>, "fields": [<post_aggregator>, <post_aggregator>, ...] }
This returns the value produced by the specified aggregator.
fieldName
refers to the output name of the aggregator given in the aggregations portion of the query.
{ "type" : "fieldAccess", "fieldName" : <aggregator_name> }
The constant post-aggregator always returns the specified value.
{ "type" : "constant", "name" : <output_name>, "value" : <numerical_value> }
Applies the provided JavaScript function to the given fields. Fields are passed as arguments to the JavaScript function in the given order.
postAggregation : { "type": "javascript", "name": <output_name>, "fieldNames" : [<aggregator_name>, <aggregator_name>, ...], "function": <javascript function> }
Example JavaScript aggregator:
{ "type": "javascript", "name": "absPercent", "fieldNames": ["delta", "total"], "function": "function(delta, total) { return 100 * Math.abs(delta) / total; }" }
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:
{ ... "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 } ] } ... }