presto-main/src/main/java/io/prestosql/operator/BigintGroupByHash.java/BigintGroupByHash/getGroupIds
class BigintGroupByHash:    @Override
    public Work<GroupByIdBlock> getGroupIds(Page page)
    {
        currentPageSizeInBytes = page.getRetainedSizeInBytes();
        return new GetGroupIdsWork(page.getBlock(hashChannel));
    }