今天学习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
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;
课后作业如下
1.以默认方式启动事务1，修改事务隔离级别，查看transaction_isolation
omm=# start transaction;
START TRANSACTION
omm=# set local transaction isolation level repeatable read read write;
SET
omm=# show transaction_isolation;
(1 row)
omm=# transaction_isolation
repeatable read
omm=# show transaction_read_only;
transaction_read_only
off
(1 row)
omm=# end;
COMMIT
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
omm=# start transaction isolation level read committed read write;
omm=# START TRANSACTION
omm=# create table product(product_id int,product_name char(30));
CREATE TABLE
omm=#
omm=# set local transaction isolation level read committed read only;
SET
omm=# insert into product values(1,‘phone’);
ERROR: cannot execute INSERT in a read-only transaction
omm=# end;
ROLLBACK
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
omm=# create table product(product_id int,product_name char(30));
omm=# CREATE TABLE
omm=# start transaction;
START TRANSACTION
omm=# show transaction_isolation;
transaction_isolation
read committed
(1 row)
omm=# show transaction_read_only;
(1 row)
omm=# transaction_read_only
off
omm=# insert into product values(1,‘computer’);
INSERT 0 1
omm=# select * from product;
product_id | product_name
------------±-------------------------------
1 | computer
(1 row)
omm=# update product set product_name=‘phone’ where product_id = 1;
UPDATE 1
omm=# select * from product;
product_id | product_name
------------±-------------------------------
1 | phone
(1 row)
omm=# savepoint my_sp1;
omm=# SAVEPOINT
omm=# insert into product values(2,‘light’);
INSERT 0 1
omm=# select * from product;
product_id | product_name
------------±-------------------------------
1 | phone
2 | light
(2 rows)
omm=# rollback to savepoint my_sp1;
ROLLBACK
omm=# select * from product;
(1 row)
omm=# product_id | product_name
------------±-------------------------------
1 | phone
omm=# release savepoint my_sp1;
omm=# RELEASE
omm=# insert into product values(3,‘mouse’);
INSERT 0 1
omm=# commit;
COMMIT
omm=# select * from product;
product_id | product_name
------------±-------------------------------
1 | phone
3 | mouse
(2 rows)
4.清理数据
omm=# drop table product;
DROP TABLE
