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