@Override
  public Optional<ExtendedCell> midKey() throws IOException {
    return Optional.ofNullable(dataBlockIndexReader.midkey(this));
  }
@return Midkey for this file. We work with block boundaries only so returned midkey is an
        approximation only.
hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/HFileReaderImpl.java