2 Database Systems versus File Systems(Cont) Drawbacks of using file systems(cont) Atomicity of updates Failures may leave database in an inconsistent state with partial updates carried out E.g. transfer of funds from one account to another should either complete or not happen at all Concurrent access by multiple users Concurrent accessed needed for performance Uncontrolled concurrent accesses can lead to inconsistencies E.g. two people reading a balance and updating it at the same time 标 Security problems Database System Concepts 1.6 @Silberschatz, Korth and Sudarshan
Database System Concepts 1.6 ©Silberschatz, Korth and Sudarshan 2 Database Systems versus File Systems(Cont.) Drawbacks of using file systems (cont.) Atomicity of updates Failures may leave database in an inconsistent state with partial updates carried out E.g. transfer of funds from one account to another should either complete or not happen at all Concurrent access by multiple users Concurrent accessed needed for performance Uncontrolled concurrent accesses can lead to inconsistencies – E.g. two people reading a balance and updating it at the same time Security problems
3 View of Data 3.1 Data Abstraction Physical level describes how a record(e.g customer)is stored Logical level: describes data stored in database, and the relationships among the data type customer record name: string; street: string city: integer; end View level; application programs hide details of data types. Views can also hide information(e.g, salary) for security purposes Database System Concepts 1.7 @Silberschatz, Korth and Sudarshan
Database System Concepts 1.7 ©Silberschatz, Korth and Sudarshan 3 View of Data 3.1 Data Abstraction Physical level describes how a record (e.g., customer) is stored. Logical level: describes data stored in database, and the relationships among the data. type customer = record name : string; street : string; city : integer; end; View level: application programs hide details of data types. Views can also hide information (e.g., salary) for security purposes
3 View of Data(Cont) An architecture for a database system view level view 2 view n 81c level level Database System Concepts 1.8 OSilberschatz. Korth and Sudarshan
Database System Concepts 1.8 ©Silberschatz, Korth and Sudarshan 3 View of Data (Cont.) An architecture for a database system
3 View of Data(Cont) 3.2 Instances and schemas Similar to types and variables in programming languages Schema -the logical structure of the database e.g., the database consists of information about a set of customers and accounts and the relationship between them) Analogous to type information of a variable in a program Physical schema: database design at the physical level Logical schema: database design at the logical level Database System Concepts 1.9 @Silberschatz, Korth and Sudarshan
Database System Concepts 1.9 ©Silberschatz, Korth and Sudarshan 3 View of Data (Cont.) 3.2 Instances and Schemas Similar to types and variables in programming languages Schema – the logical structure of the database e.g., the database consists of information about a set of customers and accounts and the relationship between them) Analogous to type information of a variable in a program Physical schema: database design at the physical level Logical schema: database design at the logical level