A question about PG Authorization

Dear Guys,

May I have a question about the authorization of psql?

By using '\z' in psql context, I can get the description of authorization about each table/view.


                  r -- SELECT
                  w -- UPDATE/DELETE
                  a -- INSERT
                  R -- RULE
               arwR -- ALL


Authoriztion switches are specified as above, but what should I do if I want to grant 'update' without 'delete' to some
table/view?


thank you



Eddy