Oracle(三)

Oracle数据字典

Central to every Oracle database （是Oracle database核心）
Describes the database and its objects （描述Databases和它的Objects）
Contains read-only tables and views （包含只读表和Views）
Stored in the SYSTEM tablespace （存储在SYSTEM tablespace）
Owned by the user SYS （属于SYS用户）
Maintained by the Oracle server （由Oracle server进行维护）
Accessed with SELECT（只允许进行SELECT操作）

Data dictionary由两部分组成
Base tables（使用sql.bsq创建）
   Stores description of the database(存储Database description)
   Created with CREATE DATABASE （创建Database的时候创建）
Data dictionary views
   Used to simplify the base table information（用于简化Base table信息）
   Accessed through public synonyms
   Created with the catalog.sql script（使用catalog.sql进行创建）

Data Dictionary Contents

The data dictionary provides information about:
• Logical and physical database structures （数据库的物理结构和逻辑结构）
• Definitions and space allocations of objects （对象的定义和空间分配情况）
• Integrity constraints （完整性约束）
• Users
• Roles
• Privileges （权限）
• Auditing （审计）

How the Data Dictionary Is Used
Primary uses:
• Oracle server uses it to find information about
 – Users
 – Schema objects
 – Storage structures
• Oracle server modifies it when a DDL statement is executed.
• Users and DBAs use it as a read-only reference for information about the database.

Data Dictionary View Categories
• Three sets of static views
• Distinguished by their scope:
 – DBA: What is in all the schemas
 – ALL: What the user can access
 – USER: What is in the user’s schema