PostgreSQL 怎样给一个用户授予只读角色?

PostgreSQL没有只读角色这个权限，但是可以通过修改用户的默认参数来达到只读的目的，如下 : 
digoal=# select usename,useconfig from pg_user where usename='digoal';
 usename | useconfig 
---------+-----------
 digoal  | 
(1 row)
digoal=# alter role digoal set default_transaction_read_only=true;
ALTER ROLE
digoal=# select usename,useconfig from pg_user where usename='digoal';
 usename |              useconfig               
---------+--------------------------------------
 digoal  | {default_transaction_read_only=true}
(1 row)
连接到digoal用户进行验证,
digoal=# \c digoal digoal
You are now connected to database "digoal" as user "digoal".
digoal=> insert into test values (1);
ERROR:  cannot execute INSERT in a read-only transaction
digoal=> delete from test ;
ERROR:  cannot execute DELETE in a read-only transaction
digoal=> drop table test ;
ERROR:  cannot execute DROP TABLE in a read-only transaction