Terran: 两个进程同时读写一个数据库?

Terran: 两个进程同时读写一个数据库?

(我用 PostgreSQL)
两个进程如果同时对一个数据库进程读写操作,是否需要显式的对数据库加锁?如果需要,应该加什么级别的锁?锁的类型?      
No,I think the rdbms can do it automaticly.
If all the read and write operation are controlled by application,I don't know how to write these application and what are the rdbms doing that time.      
Thanks a lot.