openGauss每日一练第16课（学习openGauss事务控制） | （【七七】学习内容/心得体会）

学习目标

学习openGauss事务控制
事务是用户定义的一个数据库操作序列，这些操作要么全做要么全不做，是一个不可分割的工作单位

课程学习

连接数据库

#第一次进入等待15秒
#数据库启动中...
su - omm
gsql -r

1.事务控制

–通过START TRANSACTION和BEGIN语法启动事务
-以默认方式启动事务

START TRANSACTION;
select * from pg_class limit 1;
END;

–开启一个事务，设置事务的隔离级别为READ COMMITTED，访问模式为READ ONLY

BEGIN;
SET LOCAL TRANSACTION ISOLATION LEVEL READ COMMITTED READ ONLY;
show transaction_read_only;
select * from pg_class limit 1;
create schema tpcds10;
commit;

–以隔离级别为repeatable read，读/写方式启动事务

show transaction_isolation;
START TRANSACTION ISOLATION LEVEL repeatable read READ WRITE;
show transaction_isolation;
show transaction_read_only;
select * from pg_class limit 1;
create schema tpcds10;
rollback;

–事务回滚，schema没有创建成功

\dn+ tpcds10;

2.savepoint

–保存点是事务中的一个特殊记号，它允许将那些在它建立后执行的命令全部回滚，把事务的状态恢复到保存点所在的时刻

CREATE TABLE table1(a int);
START TRANSACTION;
INSERT INTO table1 VALUES (1);

–建立保存点

SAVEPOINT my_savepoint;
INSERT INTO table1 VALUES (2);

–回滚保存点

ROLLBACK TO SAVEPOINT my_savepoint;

–删除保存点

RELEASE SAVEPOINT my_savepoint;
INSERT INTO table1 VALUES (3);
COMMIT;

–查询表的内容，会同时看到1和3,不能看到2，因为2被回滚

SELECT * FROM table1;

课程作业

1.以默认方式启动事务1，修改事务隔离级别，查看transaction_isolation

START TRANSACTION;
SET LOCAL TRANSACTION ISOLATION LEVEL READ COMMITTED;
show transaction_isolation;END;

omm=# show transaction_isolation;
transaction_isolation
-----------------------
read committed
(1 row)

omm=# 

2.以读写方式启动事务2，创建新表，修改事务为只读事务，查看transaction_read_only，并向表中插入记录

START TRANSACTION;
BEGIN;
SET LOCAL TRANSACTION ISOLATION LEVEL READ COMMITTED READ WRITE;
show transaction_isolation;
create table t1 (id int);
insert into t1 values(2021);SET LOCAL TRANSACTION ISOLATION LEVEL READ COMMITTED READ ONLY;
select * from t1;
insert into t1 values(2022);
show transcation_read_only;
select * from t1;
commit;END;

3.启动事务3，对表进行增删改查，并用到创建savepoint，回滚savepoint和删除savepoint

START TRANSACTION;
BEGIN;
create table t2(id int not null,name char(10));
insert into t2 values(11,'张三'),(12,'李四');
select * from t2;
savepoint s11;
update t2 set name='张珊' where id=11;
savepoint s12;
delete from t2 where id=12;
select * from t2;
savepoint s13;
ROLLBACK TO SAVEPOINT s12;RELEASE SAVEPOINT s11;
select * from t2;
commit;
END;

4.清理数据

drop table t1;
drop table t2;