code: func (b *lowBuilder) indexFileDir() string {
  	switch {
  	case b.high.SQLite != "":
  		return filepath.Dir(b.high.SQLite)
  	case b.high.KVFile != "":
  		return filepath.Dir(b.high.KVFile)
  	case b.high.LevelDB != "":
  		return filepath.Dir(b.high.LevelDB)
  	}
  	return ""
  }
docs: // indexFileDir returns the directory of the sqlite or kv file, or the
  // empty string.