presto-main/src/main/java/io/prestosql/operator/NoChannelGroupByHash.java/NoChannelGroupByHash/getGroupIds
class NoChannelGroupByHash:    @Override
    public Work<GroupByIdBlock> getGroupIds(Page page)
    {
        updateGroupCount(page);
        return new CompletedWork<>(new GroupByIdBlock(page.getPositionCount() > 0 ? 1 : 0, RunLengthEncodedBlock.create(BIGINT, 0L, page.getPositionCount())));
    }