Hi all, I am currently working on a project that uses Druid's QueryRunner and other druid-processing classes. It uses Druid's own classes to calculate query results. I have been testing large GroupBy queries (using v2), and it seems like parallel combining threads for GroupBy queries are only enabled on the historical level. I think it is only getting called by GroupByStrategyV2.mergeRunners() <https://github.com/apache/incubator-druid/blob/druid-0.12.1/processing/src/main/java/io/druid/query/groupby/strategy/GroupByStrategyV2.java#L335> which is only called by GroupByQueryRunnerFactory.mergeRunners() on historicals.
Are GroupByMergingQueryRunnerV2 and parallel combining threads meant for computing and merging per-segment results only, or can they also be used on the broker level? I changed the logic of my project from calling queryToolChest.mergeResults() on MergeSequence (created by providing a list of per-segment/per-server sequences) to calling queryToolChest.mergeResults() on queryRunnerFactory.mergeRunners() (where each runner returns a deserialized result sequence), and that seemed to have reduced really heavy groupby query computation time or failures by quite a lot. Or is this just a coincidence and there shouldn't be a performance difference in merging groupby query results, and the only difference could've been by parallelizing the deserialization of result sequences from sub-queries? Thanks, Jisoo
