code: func (b *batchChecker) initDupOldRowValue(ctx sessionctx.Context, t table.Table, newRows [][]types.Datum) error {
  	b.dupOldRowValues = make(map[string][]byte, len(newRows))
  	b.initDupOldRowFromHandleKey()
  	return b.initDupOldRowFromUniqueKey(ctx, newRows)
  }
docs: // initDupOldRowValue initializes dupOldRowValues which contain the to-be-updated rows from storage.