presto-main/src/main/java/io/prestosql/operator/aggregation/builder/InMemoryHashAggregationBuilder.java/InMemoryHashAggregationBuilder/processPage
class InMemoryHashAggregationBuilder:    @Override
    public Work<?> processPage(Page page)
    {
        if (aggregators.isEmpty()) {
            return groupByHash.addPage(page);
        }
        else {
            return new TransformWork<>(
                    groupByHash.getGroupIds(page),
                    groupByIdBlock -> {
                        for (Aggregator aggregator : aggregators) {
                            aggregator.processPage(groupByIdBlock, page);
                        }
                        // we do not need any output from TransformWork for this case
                        return null;
                    });
        }
    }