himanshug opened a new issue #8071: add aggregators for computing mean/average URL: https://github.com/apache/incubator-druid/issues/8071 ### Motivation I have use case of querying mean value of certain columns and need an easy and efficient way to do same. ### Proposed changes We would introduce following `DoubleMeanAggregatorFactory` implementation and other related classes e.g. `DoubleMeanAggregator` . It would work by using following well known algorithm. ``` // maintain following variables long count; double mean; // update with a value v count++; mean = mean + (v - mean)/count; // merging count = count1 + count2; mean = (mean1*count1 + mean2*count2)/count; ``` consequently a new aggregator type called `doubleMean` would be made available. ### Rationale In comparison to the alternatives, proposed implementation is most straightforward and least overhead way to get mean of a column. Alternative#1: Use `doubleSum` , `doubleCount` aggregators and use `arithmetic` or `expression` post aggregator to do the division to compute mean. It becomes tedious for system generating the druid query and mean is a very common aggregation being available out of the box. Alternative#2: Add a `MeanPostAggregator` that extracts the mean from `VarianceAggregatorCollector` maintained by `VarianceAggregator` OR add a option to `VarianceAggregatorFactory` to output mean in `finalizeComputation(obj)` method. However, this would unnecessarily maintain variables for variance. ### Operational impact None ### Test plan (optional) Changes proposed here would be easily unit testable. ### Future work (optional) Maybe a `floatMean` aggregator if some use case is too paranoid about saving few bytes at query time. However, I don't think mean is a stat that should be indexed and stored in segment so `floatMean` is not important I think.
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