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）

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
musicdb=# create user user1 identified by 'taox1110!';
CREATE ROLE
openGauss=# ALTER USER user1 SYSADMIN;
ALTER ROLE
openGauss=# \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".
--在db1库创建测试表T11
musicdb1=> create table t11(col1 char(20));
CREATE TABLE
musicdb1=> insert into t11 values('Hello openGauss! 11');
INSERT 0 1
musicdb1=> select * from t11;
         col1
----------------------
 Hello openGauss! 11
(1 row)

--连接db2库，并创建测试表t12
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 t12(col1 char(20));
CREATE TABLE
musicdb2=> insert into t12 values('Hello openGauss! 12');
INSERT 0 1
musicdb2=>  select * from t12;
         col1
----------------------
 Hello openGauss! 12
(1 row)

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

openGauss=# \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 t11;
         col1
----------------------
 Hello openGauss! 11
(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)

--og跨库访问显然是不行的，这个和mysql不一样，pg是强隔离，而mysql是弱隔离
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;
         col1
----------------------
 Hello openGauss! 11
(1 row)

musicdb1=> select * from musicdb2.public.t12;
ERROR:  cross-database references are not implemented: "musicdb2.public.t12"
LINE 1: select * from musicdb2.public.t12;
                      ^

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.t12;;
         col1
----------------------
 Hello openGauss! 12
(1 row)