概述

本文档记录openGauss 3.0.0数据库每日一练第6天课程作业，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。

课程练习

创建用户user6，分别在数据库musicdb6、数据库musicdb7创建表t11,t21

[omm@ogauss1 ~]$ gsql -d postgres -p 15400 -r
gsql ((openGauss 3.0.0 build 02c14696) compiled at 2022-04-01 18:12:34 commit 0 last mr  )
Non-SSL connection (SSL connection is recommended when requiring high-security)
Type "help" for help.

omm@local:/opt/huawei/tmp [postgres]=#create user user6 with sysadmin identified by 'zs@123456';
CREATE ROLE
omm@local:/opt/huawei/tmp [postgres]=#create database musicdb6;
CREATE DATABASE
omm@local:/opt/huawei/tmp [postgres]=#create database musicdb7;
CREATE DATABASE
omm@local:/opt/huawei/tmp [postgres]=#\c musicdb6
Non-SSL connection (SSL connection is recommended when requiring high-security)
You are now connected to database "musicdb6" as user "omm".
omm@local:/opt/huawei/tmp [musicdb6]=#create table day6_db6(id int,name char(10));
CREATE TABLE
omm@local:/opt/huawei/tmp [musicdb6]=#insert into day6_db6 values(1,'musicdb6');
INSERT 0 1
omm@local:/opt/huawei/tmp [musicdb6]=#\c musicdb7
Non-SSL connection (SSL connection is recommended when requiring high-security)
You are now connected to database "musicdb7" as user "omm".
omm@local:/opt/huawei/tmp [musicdb7]=#create table day6_db7(id int,name char(10));
CREATE TABLE
omm@local:/opt/huawei/tmp [musicdb7]=#insert into day6_db7 values(1,'musicdb7');
INSERT 0 1

user6用户连接到数据库musicdb6，并访问数据库musicdb6下的表day6_db6

[omm@ogauss1 ~]$ gsql -d postgres -p 15400 -r
gsql ((openGauss 3.0.0 build 02c14696) compiled at 2022-04-01 18:12:34 commit 0 last mr  )
Non-SSL connection (SSL connection is recommended when requiring high-security)
Type "help" for help.

omm@local:/opt/huawei/tmp [postgres]=#\c musicdb6 user6
Password for user user6: 
Non-SSL connection (SSL connection is recommended when requiring high-security)
You are now connected to database "musicdb6" as user "user6".
user6@local:/opt/huawei/tmp [musicdb6]=>\d
                          List of relations
 Schema |   Name   | Type  | Owner |             Storage              
--------+----------+-------+-------+----------------------------------
 public | day6_db6 | table | omm   | {orientation=row,compression=no}
(1 row)

user6@local:/opt/huawei/tmp [musicdb6]=>select * from musicdb6.public.day6_db6;
 id |    name    
----+------------
  1 | musicdb6  
(1 row)

访问成功。

在这个连接中（使用数据库用户user6连接到数据库musicdb6）访问数据库musicdb7下的day6_db7表(会报错)

user6@local:/opt/huawei/tmp [musicdb6]=>select * from musicdb7.public.day6_db7;
ERROR:  cross-database references are not implemented: "musicdb7.public.day6_db7"
LINE 1: select * from musicdb7.public.day6_db7;
                      ^
user6@local:/opt/huawei/tmp [musicdb6]=>

访问失败

使用数据库用户user6连接到数据库musicdb7，访问数据库musicdb7下的day6_db7表

user6@local:/opt/huawei/tmp [musicdb6]=>\c musicdb7
Password for user user6: 
Non-SSL connection (SSL connection is recommended when requiring high-security)
You are now connected to database "musicdb7" as user "user6".
user6@local:/opt/huawei/tmp [musicdb7]=>select * from musicdb7.public.day6_db7;
 id |    name    
----+------------
  1 | musicdb7  
(1 row)

访问成功。