code: func (sf *ScalarFunction) ConstItem() bool {
  	// Note: some unfoldable functions are deterministic, we use unFoldableFunctions here for simplification.
  	if _, ok := unFoldableFunctions[sf.FuncName.L]; ok {
  		return false
  	}
  	for _, arg := range sf.GetArgs() {
  		if !arg.ConstItem() {
  			return false
  		}
  	}
  	return true
  }
docs: // ConstItem implements Expression interface.