public IFileScanHandle next() {
    curIndex--;
    if (curIndex >= unSeqSize) {
      return seqFileScanHandles.get(curIndex - unSeqSize);
    } else {
      return unseqFileScanHandles.get(curIndex);
    }
  }
Iterate the list in descending order of time, as more recent entries are less likely to have
their TsFile resources downgraded
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/storageengine/dataregion/read/QueryDataSourceForRegionScan.java