private HashSet<String> scanForMergedParentRegions() throws IOException {
    HashSet<String> mergedParentRegions = new HashSet<>();
    // Null tablename means scan all of meta.
    MetaTableAccessor.scanMetaForTableRegions(this.master.getConnection(), r -> {
      List<RegionInfo> mergeParents = CatalogFamilyFormat.getMergeRegions(r.rawCells());
      if (mergeParents != null) {
        for (RegionInfo mergeRegion : mergeParents) {
          if (mergeRegion != null) {
            // This region is already being merged
            mergedParentRegions.add(mergeRegion.getEncodedName());
          }
        }
      }
      return true;
    }, null);
    return mergedParentRegions;
  }
Scan hbase:meta to get set of merged parent regions, this is a very heavy scan.
@return Return generated {@link HashSet}
hbase-server/src/main/java/org/apache/hadoop/hbase/master/hbck/HbckChore.java