code: func DecodeTableID(key kv.Key) int64 {
  	if !key.HasPrefix(tablePrefix) {
  		return 0
  	}
  	key = key[len(tablePrefix):]
  	_, tableID, err := codec.DecodeInt(key)
  	// TODO: return error.
  	terror.Log(errors.Trace(err))
  	return tableID
  }
docs: // DecodeTableID decodes the table ID of the key, if the key is not table key, returns 0.