修改配置文件postgresql.conf，设置默认的隔离级别：

vim postgresql.conf

default_transaction_isolation = ‘repeatable read’

重新加载配置:

pg_ctl -D /data/pg reload

查看默认的隔离级别:

show default_transaction_isolation;

postgres=# show default_transaction_isolation;
***(Single step mode: verify command)*******************************************
show default_transaction_isolation;
***(press return to proceed or enter x and return to cancel)********************default_transaction_isolation
-------------------------------repeatable read
(1 row)

二.  先开启事务A

务必先开启事务A， 然后在事务A开始后， 再开启事务B。

在事务B中执行完commit提交后， 再在事务A中修改同一行。

事务A操作：

test=# begin;
***(Single step mode: verify command)*******************************************
begin;
***(press return to proceed or enter x and return to cancel)********************BEGIN
test=*# select * from company where id=11;
***(Single step mode: verify command)*******************************************
select * from company where id=11;
***(press return to proceed or enter x and return to cancel)********************id | name | age |                      address                       | salary | join_date
----+------+-----+----------------------------------------------------+--------+------------11 | Paul |  66 | California                                         |  20000 | 2001-07-13
(1 row)test=*# select * from company where id=11;
***(Single step mode: verify command)*******************************************
select * from company where id=11;
***(press return to proceed or enter x and return to cancel)********************id | name | age |                      address                       | salary | join_date
----+------+-----+----------------------------------------------------+--------+------------11 | Paul |  66 | California                                         |  20000 | 2001-07-13
(1 row)test=*# select * from company where id=11;
***(Single step mode: verify command)*******************************************
select * from company where id=11;
***(press return to proceed or enter x and return to cancel)********************id | name | age |                      address                       | salary | join_date
----+------+-----+----------------------------------------------------+--------+------------11 | Paul |  66 | California                                         |  20000 | 2001-07-13
(1 row)

三. 开启事务B，修改特定行，并提交

test=# begin;
***(Single step mode: verify command)*******************************************
begin;
***(press return to proceed or enter x and return to cancel)********************BEGIN
test=*#
test=*# select * from company where id=11;
***(Single step mode: verify command)*******************************************
select * from company where id=11;
***(press return to proceed or enter x and return to cancel)********************id | name | age |                      address                       | salary | join_date
----+------+-----+----------------------------------------------------+--------+------------11 | Paul |  66 | California                                         |  20000 | 2001-07-13
(1 row)test=*# update company set age=88 where id=11;
***(Single step mode: verify command)*******************************************
update company set age=88 where id=11;
***(press return to proceed or enter x and return to cancel)********************UPDATE 1
test=*# select * from company where id=11;
***(Single step mode: verify command)*******************************************
select * from company where id=11;
***(press return to proceed or enter x and return to cancel)********************id | name | age |                      address                       | salary | join_date
----+------+-----+----------------------------------------------------+--------+------------11 | Paul |  88 | California                                         |  20000 | 2001-07-13
(1 row)test=*# commit;
***(Single step mode: verify command)*******************************************
commit;
***(press return to proceed or enter x and return to cancel)********************

四. 在事务A中， 修改同一行，造成可重复读的状态变化，查看postgres的处理

test=*# update company set age=99 where id=11;
***(Single step mode: verify command)*******************************************
update company set age=99 where id=11;
***(press return to proceed or enter x and return to cancel)********************2022-05-24 12:50:53.649 EDT [443682] ERROR:  could not serialize access due to concurrent update
2022-05-24 12:50:53.649 EDT [443682] STATEMENT:  update company set age=99 where id=11;
ERROR:  could not serialize access due to concurrent update

结论：

一. postgres对于可重复读的隔离级别，如果出现读取行的状态变化，则本事务将会提交失败并回滚

二. mysql的可重复读， 如果出现读取行的状态变化，则本事务将会覆盖先前的事务