【数据库系统】实例和模式

Relationship between schema and instance

It is similar to types and variables in programming languages.

Schema is analogous to type information of a variable in a program.（变量类型）
Instance is analogous to the value of a variable.（变量取值）

Schema（模式）

Schema is the overall design of the database.（数据库的整体设计）

Several schemas of DBS

• Physical schema（物理模式/内模式/存储模式）: database design at the physical level.
• Logical schema（逻辑模式/模式/概念模式）: database design at the logical level.
• Sub schema（子模式/外模式）: describe different views at the view level.

@see level of data abstraction

Physical data independence（物理数据独立性）

Application do not depend on physical schema, and thus need not be rewritten if the physical schema changes.

• Applications depend on the logical schema.
• Physical schema is hidden beneath the logical schema.

@See detailed analysis

Instance（实例）

An instance is the collection of information stored in the database at a particular point in time.（特定时刻存储在数据库中的信息的集合）