@drcrallen thanks, but some of my comments are not addressed. Some queries don't use `ChainedExecutionQueryRunner`. The groupBy query is an example. groupBy v1 and groupBy v2 use `GroupByMergedQueryRunner.run()` and `GroupByMergingQueryRunnerV2.run()` in `QueryRunnerFactory.mergeRunners`, respectively. And they also have some blocking mechanism which requires to be wrapped by `ForkJoinPool.managedBlock()`. For example, there are two places in `GroupByMergingQueryRunnerV2.run()`. One is to [get mergeBuffers](https://github.com/apache/incubator-druid/blob/master/processing/src/main/java/org/apache/druid/query/groupby/epinephelinae/GroupByMergingQueryRunnerV2.java#L336-L340) and another is to [wait for futures to complete](https://github.com/apache/incubator-druid/blob/master/processing/src/main/java/org/apache/druid/query/groupby/epinephelinae/GroupByMergingQueryRunnerV2.java#L365). All these places should be handled properly.
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