My postgresql version is 7.2.1, i found a user's acl information remained when i deleted the user.
login as a superuser(postgres).
create a table called films in currnet database.
create a user called filmuser.
grant select on films to filmuser.
filmuser now can select on films.
delete filmuser.
create user filmuser again with same sysid as the privious one.
i find the new filmuser can select on table films, without granting statement excuted.