presto-main/src/main/java/io/prestosql/operator/StreamingAggregationOperator.java/StreamingAggregationOperator/findNextGroupStart
class StreamingAggregationOperator:    private int findNextGroupStart(int startPosition, Page page)
    {
        for (int i = startPosition + 1; i < page.getPositionCount(); i++) {
            if (!pagesHashStrategy.rowEqualsRow(startPosition, page, i, page)) {
                return i;
            }
        }

        return page.getPositionCount();
    }