Introduction to the Data Dictionary (257)

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本文介绍了Oracle数据库中数据字典的重要性和结构。数据字典是只读的表格集合，包含了所有模式对象的定义、空间分配及使用情况、列的默认值、完整性约束等关键信息。它是数据库的核心组成部分，对从最终用户到应用程序设计师和数据库管理员的所有用户都至关重要。

One of the most important parts of an Oracle database is its data dictionary, which is a
read-only set of tables that provides information about the database. A data dictionary
contains:
■ The definitions of all schema objects in the database (tables, views, indexes,
clusters, synonyms, sequences, procedures, functions, packages, triggers, and so
on)
■ How much space has been allocated for, and is currently used by, the
schema objects
■ Default values for columns
■ Integrity constraint information
■ The names of Oracle users
■ Privileges and roles each user has been granted
■ Auditing information, such as who has accessed or updated various schema
objects
■ Other general database information
The data dictionary is structured in tables and views, just like other database data. All
the data dictionary tables and views for a given database are stored in that database’s
SYSTEM tablespace.

Not only is the data dictionary central to every Oracle database, it is an important tool
for all users, from end users to application designers and database administrators. Use
SQL statements to access the data dictionary. Because the data dictionary is read only,
you can issue only queries (SELECT statements) against it’s tables and views.

数据字典
1. 数据字典包含以下信息:所有方案对象的定义，空间分配及使用情况，列的默认值，完整性约束等
2. 数据字典是只读的

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