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）

学习目标

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

omm=# CREATE TABLESPACE music_tbs RELATIVE LOCATION 'tablespace/test_ts1';
CREATE TABLESPACE
omm=# CREATE DATABASE musicdb  WITH TABLESPACE = music_tbs;
CREATE DATABASE
omm=# CREATE DATABASE musicdb1  WITH TABLESPACE = music_tbs;
CREATE DATABASE
omm=# CREATE DATABASE musicdb2  WITH TABLESPACE = music_tbs;
CREATE DATABASE
omm=# CREATE USER user1 IDENTIFIED BY 'kunpeng@1234';
NOTICE:  The encrypted password contains MD5 ciphertext, which is not secure.
omm=# CREATE ROLE
omm=# ALTER USER user1 SYSADMIN;
ALTER ROLE

omm=# \c musicdb1 user1
Password for user user1: 
musicdb1=> Non-SSL connection (SSL connection is recommended when requiring high-security)
You are now connected to database "musicdb1" as user "user1".
musicdb1=> 
musicdb1=> create table t11(col1 char(20));
musicdb1=> CREATE TABLE
 insert into t11 values('Hello openGauss! 11');
INSERT 0 1
musicdb1=>  select * from t11;
         col1         
----------------------
 Hello openGauss! 11 
(1 row)

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(col1 char(20));
CREATE TABLE
musicdb2=>  insert into t21 values('Hello openGauss! 22');
INSERT 0 1
musicdb2=>  select * from t21;
         col1         
----------------------
 Hello openGauss! 22 
(1 row)

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

usicdb2=> \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;
         col1         
----------------------
 Hello openGauss! 11 
(1 row)

musicdb1=> \dt
                        List of relations
 Schema | Name | Type  | Owner |             Storage              
--------+------+-------+-------+----------------------------------
 public | t11  | table | user1 | {orientation=row,compression=no}
(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)

musicdb1=> \dt
                        List of relations
 Schema | Name | Type  | Owner |             Storage              
--------+------+-------+-------+----------------------------------
 public | t11  | table | user1 | {orientation=row,compression=no}
(1 row)

musicdb1=> select * from musicdb2.public.t21;
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。

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;
         col1         
----------------------
 Hello openGauss! 22 
(1 row)

musicdb2=> \dt
                        List of relations
 Schema | Name | Type  | Owner |             Storage              
--------+------+-------+-------+----------------------------------
 public | t21  | table | user1 | {orientation=row,compression=no}
(1 row)

musicdb2=> 

总结：可以使用DatabaseName.SchemaName.TableName来标识openGauss中的一个表。