presto-main/src/main/java/io/prestosql/operator/ChannelSet.java/ChannelSet/ChannelSet
class ChannelSet:    public ChannelSet(GroupByHash hash, boolean containsNull, int[] hashChannels)
    {
        this.hash = hash;
        this.containsNull = containsNull;
        this.hashChannels = hashChannels;
    }