code: func EncodeTableIndexPrefix(tableID, idxID int64) kv.Key {
  	key := make([]byte, 0, prefixLen)
  	key = appendTableIndexPrefix(key, tableID)
  	key = codec.EncodeInt(key, idxID)
  	return key
  }
docs: // EncodeTableIndexPrefix encodes index prefix with tableID and idxID.