PostgreSQL支持的隔离级别：
- serializable
- repeatable read
- read committed
- read uncommitted
PostgreSQL默认的隔离级别为read committed。
查看当前默认的隔离级别：
postgres=# show default_transaction_isolation;
default_transaction_isolation
-------------------------------
read committed
(1 row)
查看当前的隔离级别：
postgres=# show transaction_isolation;
transaction_isolation
-----------------------
read committed
(1 row)
1. 修改PostgreSQL默认的隔离级别
修改配置文件postgresql.conf，设置默认的隔离级别，如下：
vim postgresql.conf
default_transaction_isolation = 'read committed'
重新加载配置：
pg_ctl -D /data/pg reload
2. 动态修改PostgreSQL隔离级别
修改隔离级别必须在事务中执行，可以修改默认的隔离级别和当前会话的隔离级别，语法如下：
set default_transaction_isolation='repeatable read';
set transaction isolation level serializable;
示例：
postgres=# begin;
BEGIN
postgres=# set default_transaction_isolation='repeatable read';
SET
postgres=# show default_transaction_isolation;
default_transaction_isolation
-------------------------------
repeatable read
(1 row)
postgres=# begin;
BEGIN
postgres=# set transaction isolation level serializable;
SET
postgres=# show transaction_isolation;
transaction_isolation
-----------------------
serializable
(1 row)
