Execution of the query seems to be one logical step in the overall run_aggregate path. We should break it into a helper functions to clean up the high-level path, one for executing explain and one for executing a normal query. The inputs for explain are the aggregation plan, the explain verbosity, the executor, and the ReplyBuilderInterface. The inputs for normal execution are the aggregation plan, the expression context, the privilege vector, the externalDataSourceScopeGuard, the executors, and the ReplyBuilderInterface.