presto-main/src/main/java/io/prestosql/operator/StreamingAggregationOperator.java/StreamingAggregationOperator/addRowsToAggregates
class StreamingAggregationOperator:    private void addRowsToAggregates(Page page, int startPosition, int endPosition)
    {
        for (Aggregator aggregator : aggregates) {
            aggregator.processPage(page.getRegion(startPosition, endPosition - startPosition + 1));
        }
    }