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>, ...] }
In the case of a division (/
), if the denominator is 0
then 0
is returned regardless of the numerator.
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; }" }
The hyperUniqueCardinality post aggregator is used to wrap a hyperUnique object such that it can be used in post aggregations.
{ "type" : "hyperUniqueCardinality", "fieldName" : <the name field value of the hyperUnique aggregator>}
It can be used in a sample calculation as so:
"aggregations" : [{ {"type" : "count", "name" : "rows"}, {"type" : "hyperUnique", "name" : "unique_users", "fieldName" : "uniques"} }], "postAggregations" : { "type" : "arithmetic", "name" : "average_users_per_row", "fn" : "/", "fields" : [ { "type" : "hyperUniqueCardinality", "fieldName" : "unique_users" }, { "type" : "fieldAccess", "name" : "rows", "fieldName" : "rows" } ] }
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 } ] } ... }