code: func PopRowFirstArg(ctx sessionctx.Context, e Expression) (ret Expression, err error) {
  	if f, ok := e.(*ScalarFunction); ok && f.FuncName.L == ast.RowFunc {
  		args := f.GetArgs()
  		if len(args) == 2 {
  			return args[1], nil
  		}
  		ret, err = NewFunction(ctx, ast.RowFunc, f.GetType(), args[1:]...)
  		return ret, err
  	}
  	return
  }
docs: // PopRowFirstArg pops the first element and returns the rest of row.
  // e.g. After this function (1, 2, 3) becomes (2, 3).