分布式数据库：dblink
dblink是数据库对象、允许你访问远程表及执行远程程序、另外、dblink是单向连接、创建dblink时、oracle在数据字典记录其元数据.在
dblink是数据库对象、允许你访问远程表及执行远程程序、另外、dblink是单向连接、创建dblink时、Oracle在数据字典记录其元数据.
在db link时，，必须关心一下global_name.这个缺省下，是作为网络服务名对外提供服务的。但大部分我们是不用的。
global_names参数并不是用来控制global_name的，而是用来限制dblink的使用.global_names参数的作用，简单得说来就是：当global_names参数设置为TRUE时，使用dblink的名称必须与被连接库的GLOBAL_NAME一致。
dblink共有3类：用户级、数据库级、网络级
public的指定与否：
dblink远程数据库用户验证方法：
创建举例：
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