Currently all singleton rowsets with data are considered overlapping upon construction, even when the rowset contains only one segment. In the meanwhile, singleton rowsets could be input to base compaction (when the tablet hasn't been compacted for base_compaction_interval_seconds_since_last_operation) as long as they are converted into non-overlapping rowsets by cumulative compactor.
By making rowset with one segment non-overlapping, we can avoid the work to convert such rowset to non-overlapping rowset in cumulative compaction.
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