code: func (b *builtinRowCountSig) evalInt(_ chunk.Row) (res int64, isNull bool, err error) {
  	res = int64(b.ctx.GetSessionVars().StmtCtx.PrevAffectedRows)
  	return res, false, nil
  }
docs: // evalInt evals ROW_COUNT().
  // See https://dev.mysql.com/doc/refman/5.7/en/information-functions.html#function_row-count.