presto-main/src/main/java/io/prestosql/operator/AggregationOperator.java/AggregationOperatorFactory/createOperator
class AggregationOperatorFactory:        @Override
        public Operator createOperator(DriverContext driverContext)
        {
            checkState(!closed, "Factory is already closed");
            OperatorContext operatorContext = driverContext.addOperatorContext(operatorId, planNodeId, AggregationOperator.class.getSimpleName());
            return new AggregationOperator(operatorContext, step, accumulatorFactories, useSystemMemory);
        }