作业：
1.准备测试环境
su - omm
create tablespace music_tbs relative location 'tablespace/test_ts1';
create database musicdb1 with tablespace = musci_tbs;
create database musicdb2 with tablespace = music_tbs;
2.创建用户user1，分别在数据库musicdb1、数据库musicdb2创建表t11,t21
omm=# CREATE USER user1 IDENTIFIED BY 'kunpeng@1234';
NOTICE: The encrypted password contains MD5 ciphertext, which is not secure.
CREATE ROLE
omm=# ALTER USER user1 SYSADMIN;
omm=# ALTER ROLE
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));
CREATE TABLE
musicdb1=> 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));
musicdb2=> CREATE TABLE
musicdb2=> insert into t21 values('Hello openGauss! 22');
INSERT 0 1
musicdb2=> select * from t21;
col1
----------------------
Hello openGauss! 22
##六级标题(1 row)
3.user1用户连接到数据库musicdb1，并访问数据库musicdb1下的表t11
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)
4.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=> select * from musicdb2.public.t21;
ERROR: cross-database references are not implemented: "musicdb2.public.t21"
LINE 1: select * from musicdb2.public.t21;
^
5.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=>
openGauss用户不允许跨库查询
