code: func (t *tableCommon) getRollbackableMemStore(ctx sessionctx.Context) (kv.RetrieverMutator, error) {
  	if ctx.GetSessionVars().LightningMode {
  		return ctx.Txn(true)
  	}
  
  	bs := ctx.GetSessionVars().GetWriteStmtBufs().BufStore
  	if bs == nil {
  		txn, err := ctx.Txn(true)
  		if err != nil {
  			return nil, err
  		}
  		bs = kv.NewBufferStore(txn, kv.DefaultTxnMembufCap)
  	} else {
  		bs.Reset()
  	}
  	return bs, nil
  }
docs: // getRollbackableMemStore get a rollbackable BufferStore, when we are importing data,
  // Just add the kv to transaction's membuf directly.