openGauss每日一练第6天|用户一次只能连接到一个数据库

春风得意马蹄疾，一日看尽长安花
opneGaussDB21天，第6天的课程总结：
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。（注：本实验没有使用dblink插件）
当用户user1连接到数据库musicdb1上时，没法访问数据库musicdb2上的表。

作业答案：
数据准备：

su - omm
gsql -r

omm=# CREATE TABLESPACE music_tbs RELATIVE LOCATION 'tablespace/test_ts1';
omm=# CREATE DATABASE musicdb1  WITH TABLESPACE = music_tbs;
omm=# CREATE DATABASE musicdb2  WITH TABLESPACE = music_tbs;

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

–执行下面的SQL语句，创建用户user1：

CREATE USER user1 IDENTIFIED BY 'kunpeng@1234';

–授予user1数据库系统的SYSADMIN权限：

ALTER USER user1 SYSADMIN;

–用户user1登录到数据库musicdb1，创建表t88，并插入、查看数据

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 t88(col1 char(20));
CREATE TABLE
musicdb1=> insert into t88 values('Hello openGauss! 88');
musicdb1=> INSERT 0 1

musicdb1=>  select * from t88;
(1 row)

musicdb1=>          col1         
----------------------
 Hello openGauss! 88 

–用户user2登录到数据库musicdb1，创建表t99，并插入、查看数据

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 t99(col1 char(20));
musicdb2=> CREATE TABLE
insert into t99 values('Hello openGauss! 99');
INSERT 0 1
musicdb2=> select * from t99;
(1 row)

musicdb2=>          col1         
----------------------
 Hello openGauss! 99 

2.user1用户连接到数据库musicdb1，并访问数据库musicdb1下的表t88

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.t88;
         col1         
----------------------
 Hello openGauss! 88 
(1 row)

musicdb1=> 

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t99表(会报错)
报错信息如下:

musicdb1=> select * from musicdb2.public.t99;
ERROR:  cross-database references are not implemented: "musicdb2.public.t99"
LINE 1: select * from musicdb2.public.t99;

4.使用数据库用户user1连接到数据库musicdb2，访问数据库musicdb2下的t99表。
–可以正常访问，数据库中的表是存在的。但不能跨库访问。

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.t99;
         col1         
----------------------
 Hello openGauss! 99 
(1 row)

musicdb2=>