code: func (sf *ScalarFunction) Equal(ctx sessionctx.Context, e Expression) bool {
  	fun, ok := e.(*ScalarFunction)
  	if !ok {
  		return false
  	}
  	if sf.FuncName.L != fun.FuncName.L {
  		return false
  	}
  	return sf.Function.equal(fun.Function)
  }
docs: // Equal implements Expression interface.