presto-main/src/main/java/io/prestosql/operator/AggregationOperator.java/AggregationOperator/AggregationOperatorFactory
class AggregationOperator:        public AggregationOperatorFactory(int operatorId, PlanNodeId planNodeId, Step step, List<AccumulatorFactory> accumulatorFactories, boolean useSystemMemory)
        {
            this.operatorId = operatorId;
            this.planNodeId = requireNonNull(planNodeId, "planNodeId is null");
            this.step = step;
            this.accumulatorFactories = ImmutableList.copyOf(accumulatorFactories);
            this.useSystemMemory = useSystemMemory;
        }