code: func NewIndex(physicalID int64, tblInfo *model.TableInfo, indexInfo *model.IndexInfo) table.Index {
  	index := &index{
  		idxInfo: indexInfo,
  		tblInfo: tblInfo,
  		// The prefix can't encode from tblInfo.ID, because table partition may change the id to partition id.
  		prefix: tablecodec.EncodeTableIndexPrefix(physicalID, indexInfo.ID),
  	}
  	return index
  }
docs: // NewIndex builds a new Index object.