最新OCP考试071真题解析（2020年版）
Which two statements are true about data dictionary?
A) The data dictionary does not store metadata in tables.
B) Views with the prefix DBA_ display only metadata for objects in the SYS schema.
C) Views with the prefix ALL , DBA and USER__ are not all available for every type of metadata.
D) The data dictionary is accessible when the database is closed.
E) Views with the prefix ALL__ display metadata for objects to which the current user has access.
一、Oracle Data Dictionary(oracle数据字典)
An important part of an Oracle database is its data dictionary, which is a read-only set of tables that provides administrative metadata about the database. A data dictionary contains information such as the following:
Oracle数据库中有一块重要的部分是oracle数据字典，oracle数据字典是一组只读表的集合，它们存储oracle数据库的管理元数据。
数据字典包含以下信息：
The definitions of every schema object in the database, including default values for columns and integrity constraint information.
The amount of space allocated for and currently used by the schema objects.
The names of Oracle Database users, privileges and roles granted to users, and auditing information related to users.
数据库中每个用户（模式）的定义，包括列的默认值和完整性约束信息
为模式对象分配的空间量以及当前已使用的空间量
Oracle数据库用户的名称，授予用户的特权和角色以及与用户有关的审核信息。
The data dictionary is a central part of data management for every Oracle database. For example, the database performs the following actions:
Accesses the data dictionary to find information about users, schema objects, and storage structures.
Modifies the data dictionary every time that a DDL statement is issued.
数据字典是每个Oracle数据库数据管理的核心部分。例如，数据库执行以下操作：
访问数据字典以查找有关用户，架构对象和存储结构的信息
每次发出DDL语句时修改数据字典。
Because Oracle Database stores data dictionary data in tables, just like other data, users can query the data with SQL. For example, users can run SELECT statements to determine their privileges, which tables exist in their schema, which columns are in these tables, whether indexes are built on these columns, and so on.
因为Oracle数据库像其他数据一样将数据字典数据存储在表中，所以用户可以使用SQL查询数据。例如，用户可以运行SELECT语句来确定其特权，其模式中存在哪些表，这些表中的哪些列，是否在这些列上建立索引等等。
二、Contents of the Data Dictionary（数据字典的内容）
The data dictionary consists of the following types of objects:
- Base tables （基础表）
These underlying tables store information about the database. Only Oracle Database should write to and read these tables. Users rarely access the base tables directly because they are normalized and most data is stored in a cryptic format.
这些基础表存储有关数据库的信息。 只有Oracle数据库可写入和读取这些表。数据库的用户很少可以直接访问基表，因为它们已被规范化并且大多数数据以加密格式存储。
- Views（视图）
These views decode the base table data into useful information, such as user or table names, using joins and WHERE clauses to simplify the information. These views contain the names and description of all objects in the data dictionary. Some views are accessible to all database users, whereas others are intended for administrators only.
这些视图可以通过连接（join）和WHERE子句从基本表查询获取到对用户有用的信息（内部逻辑数据解码操作），这些有用的信息包括：用户名、表名、表的描述。
有些视图可以供oracle数据库用户查询访问，而有些视图仅数据库管理员才可以查询访问。
Typically, data dictionary views are grouped in sets. In many cases, a set consists of three views containing similar information and distinguished from each other by their prefixes, as shown in Table 6-1. By querying the appropriate views, you can access only the information relevant for you.
通常，数据字典视图按组分组。 在许多情况下，一个集合由三个包含相似信息并通过其前缀彼此区分的视图组成，如表6-1所示。 通过查询适当的视图，您只能访问与您相关的信息。
Table 6-1 Data Dictionary View Sets
| Prefix |
User Access |
Contents |
Notes |
| DBA_ |
Database administrators |
All objects 所有对象 |
Some DBA_ views have additional columns containing information useful to the administrator. 一些DBA_视图具有其他列，其中包含对管理员有用的信息。 |
| ALL_ |
All users |
Objects to which user has privileges 对用户有权限的对象 |
Includes objects owned by user. These views obey the current set of enabled roles. 包括用户拥有的对象。 这些视图遵循当前启用的角色集。 |
| USER_ |
All users |
Objects owned by user 属于用户的对象 |
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. 带有前缀USER_的视图通常不包括列OWNER。 USER_视图中隐含此列是发出查询的用户。 |
Not all views sets have three members. For example, the data dictionary contains a DBA_LOCK view but no ALL_LOCK view.
并非所有视图集都有以上三个前缀， 例如：数据字典包含DBA_LOCK视图，但不包含ALL_LOCK视图。