openGauss每日一练第6天 用户一次只能连接到一个数据库，没法访问其他数据库的对象

学习目标

学习openGauss体系结构，通过实验，了解用户一次只能连接到一个数据库，没法访问其他数据库的对象。（在线环境没有dblink插件）

课堂学习

实验当用户user1连接到数据库musicdb1上时，没法访问数据库musicdb2上的表。

课后作业

1.创建用户user1，分别在数据库musicdb1、数据库musicdb2创建表t11,t21

--创建用户user1：
omm=# CREATE USER user1 IDENTIFIED BY 'meiriyilian@1234';
NOTICE:  The encrypted password contains MD5 ciphertext, which is not secure.
CREATE ROLE
--授予user1数据库系统的SYSADMIN权限：
omm=# ALTER USER user1 SYSADMIN;
ALTER ROLE

--创建表空间、数据库
/*
DROP DATABASE  IF EXISTS  musicdb1;
CREATE TABLESPACE music_tbs RELATIVE LOCATION 'tablespace/test_ts1';
CREATE DATABASE musicdb1 WITH TABLESPACE = music_tbs;
*/
omm=# DROP DATABASE  IF EXISTS  musicdb1;
DROP DATABASE
omm=# DROP DATABASE  IF EXISTS  musicdb2 ;
DROP DATABASE
omm=# CREATE TABLESPACE music_tbs RELATIVE LOCATION 'tablespace/test_ts1';
CREATE TABLESPACE 
omm=# CREATE DATABASE musicdb1 WITH TABLESPACE = music_tbs;
CREATE DATABASE
omm=# CREATE DATABASE musicdb2 WITH TABLESPACE = music_tbs;
CREATE DATABASE

--用户user1登录到数据库musicdb1，创建表t11，并插入、查看数据
/*
\c musicdb1 user1
create table t11(id integer, comm char(20));
insert into t11 values(1, 'Hello world!');
select * from t11;
*/
omm=# \c musicdb1 user1
Password for user user1: 
Non-SSL connection (SSL connection is recommended when requiring high-security)
You are now connected to database "musicdb1" as user "user1".
musicdb1=> create table t11(id integer, comm char(20));
CREATE TABLE
musicdb1=> insert into t11 values(1, 'Hello world!');
INSERT 0 1
musicdb1=> select * from t11;
musicdb1=>  id |         comm         
----+----------------------
  1 | Hello world!        
(1 row)

--用户user1登录到数据库musicdb2，创建表t21，并插入、查看数据
/*
\c musicdb2 user1
create table t21(id integer, comm char(20));
insert into t21 values(1, 'Hello world!');
select * from t21;
*/
musicdb1=> \c musicdb2 user1
Password for user user1: 
Non-SSL connection (SSL connection is recommended when requiring high-security)
You are now connected to database "musicdb2" as user "user1".
musicdb2=> create table t21(id integer, comm char(20));
CREATE TABLE
musicdb2=> insert into t21 values(1, 'Hello world!');
INSERT 0 1
musicdb2=> select * from t21;
 id |         comm         
----+----------------------
  1 | Hello world!        
(1 row)

2.user1用户连接到数据库musicdb1，并访问数据库musicdb1下的表t11

--使用user1用户连接到数据库musicdb1，访问数据库musicdb1下的表t11
musicdb2=> \c musicdb1 user1
Password for user user1: 
Non-SSL connection (SSL connection is recommended when requiring high-security)
You are now connected to database "musicdb1" as user "user1".
musicdb1=> select * from musicdb1.public.t11;
 id |         comm         
----+----------------------
  1 | Hello world!        
(1 row

3.在这个连接中（使用数据库用户user1连接到数据库musicdb1）访问数据库musicdb2下的t21表(会报错)

--使用数据库用户user1连接到数据库musicdb1，并尝试访问数据库musicdb2下的表t21
musicdb1=> \c musicdb1 user1
Password for user user1: 
Non-SSL connection (SSL connection is recommended when requiring high-security)
You are now connected to database "musicdb1" as user "user1".
musicdb1=> select * from musicdb2.public.t21;
musicdb1=> ERROR:  cross-database references are not implemented: "musicdb2.public.t21"
LINE 1: select * from musicdb2.public.t21;
                      ^

4.使用数据库用户user1连接到数据库musicdb2，访问数据库musicdb2下的t21表。

--使用user1用户连接到数据库musicdb2，访问数据库musicdb2下的表t21

musicdb1=> \c musicdb2 user1
Password for user user1: 
Non-SSL connection (SSL connection is recommended when requiring high-security)
You are now connected to database "musicdb2" as user "user1".
musicdb2=> select * from musicdb2.public.t21;
 id |         comm         
----+----------------------
  1 | Hello world!        
(1 row)

--证实用户一次只能连接到一个数据库，没法访问其他数据库的对象，即不能跨库访问。