The views of the data dictionary serve as a reference for all database users. Access the
data dictionary views with SQL statements. Some views are accessible to all Oracle
users, and others are intended for database administrators only.

The data dictionary is always available when the database is open. It resides in the
SYSTEM tablespace, which is always online.

The data dictionary consists of sets of views. In many cases, a set consists of three
views containing similar information and distinguished from each other by their
prefixes:
USER : User’s view (what is in the user’s schema)
ALL : Expanded user’s view (what the user can access)
DBA : Database administrator’s view (what is in all users’ schemas)

The set of columns is identical across views, with these exceptions:
■ Views with the prefix USER usually exclude the column OWNER. This column is
implied in the USER views to be the user issuing the query.
■ Some DBA views have additional columns containing information useful to the
administrator.

如何使用数据字典
1. 一组视图由三个包含相似信息的视图构成，其中的区别在于视图名称的前缀
USER: 用户视图（用户方案内的对象）
ALL : 扩展的用户视图（用户有权限访问的对象）
DBA : DBA 视图 (所有用户方案内的对象)
2. 同一组中各视图所包含的列是相同的，但有以下例外：
* 使用 USER 前缀的视图通常没有 OWNER 列。对于 USER 视图来说，这列的值等于提交查询的用户名。
* 有些以 DBA 为前缀的视图包含管理员所需的附加列。