presto-main/src/main/java/io/prestosql/operator/Aggregator.java/Aggregator/Aggregator
class Aggregator:    Aggregator(AccumulatorFactory accumulatorFactory, AggregationNode.Step step)
    {
        if (step.isInputRaw()) {
            intermediateChannel = -1;
            aggregation = accumulatorFactory.createAccumulator();
        }
        else {
            checkArgument(accumulatorFactory.getInputChannels().size() == 1, "expected 1 input channel for intermediate aggregation");
            intermediateChannel = accumulatorFactory.getInputChannels().get(0);
            aggregation = accumulatorFactory.createIntermediateAccumulator();
        }
        this.step = step;
    }