作者：马顺华
从事运维管理工作多年，目前就职于某科技有限公司，熟悉运维自动化、OceanBase部署运维、MySQL 运维以及各种云平台技术和产品。并已获得OceanBase认证OBCA、OBCP证书。
第6天 | openGauss中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。（注：本实验没有使用dblink插件）
课程学习
当用户user1连接到数据库musicdb1上时，没法访问数据库musicdb2上的表。
1.测试环境准备：
su - omm
gsql -r
1）––进入数据库omm
root@modb:~# su - omm
omm@modb:~$ gsql -r
gsql ((openGauss 3.0.0 build 02c14696) compiled at 2022-04-01 18:12:00 commit 0 last mr )
Non-SSL connection (SSL connection is recommended when requiring high-security)
Type "help" for help.
2）––删除指定的数据库
drop DATABASE IF EXISTS musicdb;
drop DATABASE IF EXISTS musicdb1;
drop DATABASE IF EXISTS musicdb2;
drop DATABASE IF EXISTS musicdb3;
drop tablespace IF EXISTS music_tbs;
omm=# drop DATABASE IF EXISTS musicdb;
NOTICE: database "musicdb" does not exist, skipping
DROP DATABASE
omm=# drop DATABASE IF EXISTS musicdb1;
NOTICE: database "musicdb1" does not exist, skipping
DROP DATABASE
omm=# drop DATABASE IF EXISTS musicdb2;
NOTICE: database "musicdb2" does not exist, skipping
DROP DATABASE
omm=# drop DATABASE IF EXISTS musicdb3;
NOTICE: database "musicdb3" does not exist, skipping
DROP DATABASE
omm=# drop tablespace IF EXISTS music_tbs;
NOTICE: Tablespace "music_tbs" does not exist, skipping.
DROP TABLESPACE
3）––创建表空间、测试数据库
CREATE TABLESPACE music_tbs RELATIVE LOCATION ‘tablespace/test_ts1’;
CREATE DATABASE musicdb WITH TABLESPACE = music_tbs;
CREATE DATABASE musicdb1 WITH TABLESPACE = music_tbs;
CREATE DATABASE musicdb2 WITH TABLESPACE = music_tbs;
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
4）––执行下面的SQL语句，创建用户user1：
CREATE USER user1 IDENTIFIED BY ‘kunpeng@1234’;
omm=# CREATE USER user1 IDENTIFIED BY 'kunpeng@1234';
NOTICE: The encrypted password contains MD5 ciphertext, which is not secure.
omm=# CREATE ROLE
omm=#
5）––授予user1数据库系统的SYSADMIN权限：
ALTER USER user1 SYSADMIN;
omm=# ALTER USER user1 SYSADMIN;
ALTER ROLE
omm=#
6）––用户user1登录到数据库musicdb1，创建表t11，并插入、查看数据
\c musicdb1 user1
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(col1 char(20));
insert into t11 values(‘Hello openGauss! 11’);
select * from 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)
musicdb1=>
7）––用户user1登录到数据库musicdb2，创建表t21，并插入、查看数据
\c musicdb2 user1
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));
insert into t21 values(‘Hello openGauss! 22’);
select * from t21;
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)
musicdb2=>
2.使用user1用户连接到数据库musicdb1，并访问数据库musicdb1下的表t11：
可以使用DatabaseName.SchemaName.TableName来标识openGauss中的一个表。
1）––使用user1用户连接到数据库musicdb1，访问数据库musicdb1下的表t11
\c musicdb1 user1
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=>
2）––访问数据库musicdb1下的表t11
select * from musicdb1.public.t11;
musicdb1=> select * from musicdb1.public.t11;
col1
----------------------
Hello openGauss! 11
(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），如果试图访问数据库musicdb2下的t21表会报错：
select * from musicdb2.public.t21;
ERROR: cross-database references are not implemented: “musicdb2.public.t21”
LINE 1: select * from musicdb2.public.t21;
musicdb1=> select * from musicdb2.public.t21;
ERROR: cross-database references are not implemented: "musicdb2.public.t21"
LINE 1: select * from musicdb2.public.t21;
^
musicdb1=>
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：
可以正常访问，数据库中的表是存在的。但不能跨库访问。
1）––使用数据库用户user1连接到数据库musicdb2
\c musicdb2 user1
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=>
2）––访问数据库musicdb2下的t21表
select * from musicdb2.public.t21;
musicdb2=> select * from musicdb2.public.t21;
musicdb2=> col1
----------------------
Hello openGauss! 22
(1 row)
musicdb2=>
课程总结
1.创建用户user1，分别在数据库musicdb1、数据库musicdb2创建表t11,t21
2.user1用户连接到数据库musicdb1，并访问数据库musicdb1下的表t11
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)
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。
