学习目标：学习openGauss视图
课程学习：
视图与基本表不同，是一个虚拟的表。数据库中仅存放视图的定义，而不存放视图对应的数据，这些数据仍存放在原来的基本表中。
学习内容部分如下:
课程作业：
1.为系统表PG_DATABASE创建视图，重命名视图并修改owner为jim,
omm=# create view view_pg as select * from pg_database;
CREATE VIEW
omm=# create user jim password 'password_0';
NOTICE: The encrypted password contains MD5 ciphertext, which is not secure.
CREATE ROLE
omm=# alter view view_pg owner to jim;
ALTER VIEW
2.创建一个用户表student，并在用户表上创建视图，修改视图schema;
omm=# create table student(id int,name char(10));
CREATE TABLE
omm=# create view view_student as select id,name from student;
CREATE VIEW
omm=# alter view view_student set schema tpcds;
ALTER VIEW
3.使用pg_views查看视图信息
omm=# select * from pg_views where schemaname = 'tpcds';
schemaname | viewname | viewowner | definition
------------+--------------+-----------+-----------------------------------------------
tpcds | view_student | omm | SELECT student.id, student.name FROM student;
(1 row)
4.删除视图、表、用户
omm=# drop view view_pg;
DROP VIEW
omm=# drop view tpcds.view_student;
DROP VIEW
omm=# drop table student;
DROP TABLE
omm=# drop user jim;
DROP ROLE
omm=# drop schema tpcds cascade;
NOTICE: drop cascades to 2 other objects
DETAIL: drop cascades to table tpcds.ship_mode_t1
drop cascades to table tpcds.customer
DROP SCHEMA
