@Override
  public ExtendedCell next() {
    try { // try to get next
      if (!closed && heap != null) {
        return heap.next();
      }
    } catch (IOException ie) {
      throw new IllegalStateException(ie);
    }
    return null;
  }
The MemStoreMergerSegmentsIterator extends MemStoreSegmentsIterator and performs the scan for
simple merge operation meaning it is NOT based on SQM
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/MemStoreMergerSegmentsIterator.java