presto-main/src/main/java/io/prestosql/operator/GroupedTopNBuilder.java/GroupedTopNBuilder/processPage
class GroupedTopNBuilder:    public Work<?> processPage(Page page)
    {
        return new TransformWork<>(
                groupByHash.getGroupIds(page),
                groupIds -> {
                    processPage(page, groupIds);
                    return null;
                });
    }