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96 lines
2.9 KiB
Markdown
96 lines
2.9 KiB
Markdown
---
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layout: default
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---
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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.
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There are several post-aggregators available.
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### Arithmetic post-aggregator
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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.
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Supported functions are `+`, `-`, `*`, and `/`
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The grammar for an arithmetic post aggregation is:
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<code>postAggregation : {
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"type" : "arithmetic",
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"name" : <output_name>,
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"fn" : <arithmetic_function>,
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"fields": [<post_aggregator>, <post_aggregator>, ...]
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}</code>
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### Field accessor post-aggregator
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This returns the value produced by the specified [aggregator|Aggregations](aggregator|Aggregations.html).
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`fieldName` refers to the output name of the aggregator given in the [aggregations|Aggregations](aggregations|Aggregations.html) portion of the query.
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<code>field_accessor : {
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"type" : "fieldAccess",
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"fieldName" : <aggregator_name>
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}</code>
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### Constant post-aggregator
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The constant post-aggregator always returns the specified value.
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<code>constant : {
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"type" : "constant",
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"name" : <output_name>,
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"value" : <numerical_value>,
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}</code>
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### Example Usage
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In this example, let’s calculate a simple percentage using post aggregators. Let’s imagine our data set has a metric called “total”.
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The format of the query JSON is as follows:
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<code>
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{
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...
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"aggregations" : [
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{
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"type" : "count",
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"name" : "rows"
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},
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{
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"type" : "doubleSum",
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"name" : "tot",
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"fieldName" : "total"
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}
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],
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"postAggregations" : {
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"type" : "arithmetic",
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"name" : "average",
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"fn" : "*",
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"fields" : [
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{
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"type" : "arithmetic",
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"name" : "div",
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"fn" : "/",
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"fields" : [
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{
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"type" : "fieldAccess",
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"name" : "tot",
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"fieldName" : "tot"
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},
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{
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"type" : "fieldAccess",
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"name" : "rows",
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"fieldName" : "rows"
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}
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]
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},
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{
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"type" : "constant",
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"name": "const",
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"value" : 100
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}
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]
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}
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...
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}
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</code>
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