presto-main/src/main/java/io/prestosql/operator/NoChannelGroupByHash.java/NoChannelGroupByHash/addPage
class NoChannelGroupByHash:    @Override
    public Work<?> addPage(Page page)
    {
        updateGroupCount(page);
        // create a dump work whose result is never used.
        return new CompletedWork<>(0);
    }