code: private boolean checkNewUniqueIndex(Index newIndex) {
          Table table = (Table) newIndex.getParent();
          CatalogMap<Index> existingIndexes = m_originalIndexesByTable.get(table.getTypeName());
          for (Index existingIndex : existingIndexes) {
              if (indexCovers(newIndex, existingIndex)) {
                  return true;
              }
          }
          return false;
      }
docs: Check if there is a unique index that exists in the old catalog
  that is covered by the new index. That would mean adding this index
  can't fail with a duplicate key.
  
  @param newIndex The new index to check.
  @return True if the index can be created without a chance of failing.