【云贝教育】PostgreSQL PGCA题库解析-1

psql元命令\du和\dg都可以列出角色或用户，请问这两个命令是否等价？

A.等价

B.不等价

C.不好说

D.不确定

参考答案：A

解析：

要确认某个封装命令是否等价，只需要看后台调用的SQL即可。

一、开启SQL跟踪功能

1.1 修改参数，开启sql跟踪

修改前
#log_statement = 'none'                   # none, ddl, mod, all
修改后
log_statement = 'all'                   # none, ddl, mod, all

1.2 重载使参数生效

[postgres@ora19c02 data]$ pg_ctl reload -D  $PGDATA

二、测试功能

2.1 du命令

[postgres@ora19c02 data]$ psql -d testdb
psql (15.4)
Type "help" for help.
testdb=# \du
List of roles
Role name |                         Attributes                   | Member of
-----------+------------------------------------------------------------+-------
postgres  | Superuser, Create role, Create DB, Replication, Bypass RLS | {}
test      |                                                            | {}

查看日志输出

2023-10-23 09:42:57.547 CST [56704] LOG:  statement: SELECT r.rolname, r.rolsuper, r.rolinherit,
          r.rolcreaterole, r.rolcreatedb, r.rolcanlogin,
          r.rolconnlimit, r.rolvaliduntil,
          ARRAY(SELECT b.rolname
                FROM pg_catalog.pg_auth_members m
                JOIN pg_catalog.pg_roles b ON (m.roleid = b.oid)
                WHERE m.member = r.oid) as memberof
        , r.rolreplication
        , r.rolbypassrls
        FROM pg_catalog.pg_roles r
        WHERE r.rolname !~ '^pg_'
        ORDER BY 1; 作者：云贝教育 https://www.bilibili.com/read/cv27545596/ 出处：bilibili

2.2 du命令

testdb=# \dg
List of roles
Role name |                         Attributes                         | Member of
-----------+------------------------------------------------------------+-------
postgres  | Superuser, Create role, Create DB, Replication, Bypass RLS | {}
test      |                                                            | {}

查看日志输出

2023-10-23 09:43:30.543 CST [56704] LOG:  statement: SELECT r.rolname, r.rolsuper, r.rolinherit,
r.rolcreaterole, r.rolcreatedb, r.rolcanlogin,
r.rolconnlimit, r.rolvaliduntil,
ARRAY(SELECT b.rolname
FROM pg_catalog.pg_auth_members m
JOIN pg_catalog.pg_roles b ON (m.roleid = b.oid)
WHERE m.member = r.oid) as memberof
, r.rolreplication
, r.rolbypassrls
FROM pg_catalog.pg_roles r
WHERE r.rolname !~ '^pg_'
ORDER BY 1;

三、结论

由上述实验可以得出结论，两者调用的是同一个SQL，是等价的。