presto-main/src/main/java/io/prestosql/operator/ChannelSet.java/ChannelSet/ChannelSetBuilder
class ChannelSet:        public ChannelSetBuilder(Type type, Optional<Integer> hashChannel, int expectedPositions, OperatorContext operatorContext, JoinCompiler joinCompiler)
        {
            List<Type> types = ImmutableList.of(type);
            this.hash = createGroupByHash(
                    types,
                    HASH_CHANNELS,
                    hashChannel,
                    expectedPositions,
                    isDictionaryAggregationEnabled(operatorContext.getSession()),
                    joinCompiler,
                    this::updateMemoryReservation);
            this.nullBlockPage = new Page(type.createBlockBuilder(null, 1, UNKNOWN.getFixedSize()).appendNull().build());
            this.operatorContext = requireNonNull(operatorContext, "operatorContext is null");
            this.localMemoryContext = operatorContext.localUserMemoryContext();
        }