code: func builtinPow(args []types.Datum, ctx context.Context) (d types.Datum, err error) {
  	sc := ctx.GetSessionVars().StmtCtx
  	x, err := args[0].ToFloat64(sc)
  	if err != nil {
  		return d, errors.Trace(err)
  	}
  
  	y, err := args[1].ToFloat64(sc)
  	if err != nil {
  		return d, errors.Trace(err)
  	}
  	d.SetFloat64(math.Pow(x, y))
  	return d, nil
  }
docs: // See http://dev.mysql.com/doc/refman/5.7/en/mathematical-functions.html#function_pow