ektravel commented on code in PR #14497: URL: https://github.com/apache/druid/pull/14497#discussion_r1247202415
########## docs/querying/aggregations.md: ########## @@ -422,6 +389,121 @@ It is not possible to determine a priori how well this aggregator will behave fo For these reasons, we have deprecated this aggregator and recommend using the DataSketches Quantiles aggregator instead for new and existing use cases, although we will continue to support Approximate Histogram for backwards compatibility. + +## Expression aggregators + +### Expression aggregator + +Query time only aggregator that can aggregate results using [Druid expressions](./math-expr.md) functions to facilitate building custom functions. + +| property | description | required | +| --- | --- | --- | +| `type` | must be `expression` | true | +| `name` | aggregator output name | true | +| `fields` | list of aggregator input columns | true | +| `accumulatorIdentifier` | variable which identifies the accumulator value in the `fold` and `combine` expressions | false (default `__acc`)| +| `fold` | expression to accumulate values from `fields`. The result of the expression will be stored in `accumulatorIdentifier` and available to the next computation. | true | +| `combine` | expression to combine the results of various `fold` expressions of each segment when merging results. If not defined and `fold` has a single input column in `fields`, then the `fold` expression may be used, otherwise the input is available to the expression as the `name`| false (default to `fold` expression if and only if the expression has a single input in `fields`)| +| `compare` | comparator expression which can only refer to 2 input variables, `o1` and `o2`, where `o1` and `o2` are the output of `fold` or `combine` expressions, and must adhere to the Java comparator contract. If not set, this will try to fall back to an output type appropriate comparator | false | +| `finalize` | finalize expression which can only refer to a single input variable, `o`, and is used to perform any final transformation of the output of `fold` or `combine` expressions. If not set, then the value is not transformed | false | +| `initialValue` | initial value of the accumulator for `fold` (and `combine`, if `InitialCombineValue` is null) expression | true | +| `initialCombineValue` | initial value of the accumulator for `combine` expression | false (default `initialValue`) | +| `isNullUnlessAggregated` | indicates that the default output value should be `null` if the aggregator does not process any rows. If true, the value is `null`, if false, the result of running the expressions with initial values is used instead. | false (defaults to value of `druid.generic.useDefaultValueForNull`)| +| `shouldAggregateNullInputs` | indicates if the `fold` expression should operate on any `null` input values | false (default value is `true`) | +| `shouldCombineAggregateNullInputs` | indicates if the `combine` expression should operate on any `null` input values | false (default value is `shouldAggregateNullInputs`) | Review Comment: ```suggestion | `shouldCombineAggregateNullInputs` | Indicates if the `combine` expression should operate on any `null` input values. | false (defaults to `shouldAggregateNullInputs`) | ``` -- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. To unsubscribe, e-mail: [email protected] For queries about this service, please contact Infrastructure at: [email protected] --------------------------------------------------------------------- To unsubscribe, e-mail: [email protected] For additional commands, e-mail: [email protected]
