Application designers, application developers, and database administrators can
choose appropriate isolation levels for different transactions, depending on the
application and workload. You can set the isolation level of a transaction by using one
of these statements at the beginning of a transaction:
SET TRANSACTION ISOLATION LEVEL READ COMMITTED;
SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;
SET TRANSACTION READ ONLY;
To save the networking and processing cost of beginning each transaction with a SET
TRANSACTION statement, you can use the ALTER SESSION statement to set the
transaction isolation level for all subsequent transactions:
ALTER SESSION SET ISOLATION_LEVEL SERIALIZABLE;
ALTER SESSION SET ISOLATION_LEVEL READ COMMITTED;

设置隔离级别
1. 设定事务的隔离级别 : set transactions isolation level ...
2. 使用 ALTER SESSION 语句设置一个会话所有内事务的默认隔离级别 ,
来减少网络和处理的开销