code: func (b *batchChecker) batchGetOldValues(ctx sessionctx.Context, batchKeys []kv.Key) error {
  	txn, err := ctx.Txn(true)
  	if err != nil {
  		return err
  	}
  	values, err := txn.BatchGet(batchKeys)
  	if err != nil {
  		return err
  	}
  	for k, v := range values {
  		b.dupOldRowValues[k] = v
  	}
  	return nil
  }
docs: // batchGetOldValues gets the values of storage in batch.