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DATABASE SYSTEM OVERVIEW
1. Database System (DBS)
• Definition:
Database
Management
System
(DBMS)
+
Database.
• DBMS:
Computer
Software
for
storage,
access,
and
manipulation of data.
• Database: Collection of data stored more or less permanently
on some secondary storage.
Database Schema: Structure of data (metadata). Physically stored
in "Data dictionary"
Database Instance: The collection of data stored in the database at
a particular moment. Physically stored in one or more "data file" or
"data chunk"
• DBMS developer: person or a group of people who design
and implement DBMS software
• Database Administrator (DBA): person or a group of
people who define, modify, and manage a database using an
existing DBMS
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• Database User (Naive User, Sophisticated User, and DB
Application Programmer): Person who uses a DBS
User Interface Program
Database Application Program
DBMS
Database
System
Database
Figure 1 Database System
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2. Why We Need DBS? (Historical Perspective)
• Early 60s: First Generation DBS (Simple File System)
Program1
Program2
Program3
File System of the Operating System
File1
File2
File3
File4
Figure 2 File Systems
User: Programmer (System programmer)
Problems:
Data Redundancy and Inconsistency
Data Sharing
Atomic transaction (Crash Recovery)
Concurrent-Access
Security Problems
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• Late 60s - early 70s: Second Generation DBS
Program1
Program2
Program3
Generalized File Management System
(GFMS) e.g., ASI_ST, MARK IV, Data Analyzer
DBMS
File System of the Operating System
File1
File2
File3
File4
Figure 3 Second Generation DBS
Uniform Interface
GFMS provides Database Language
Data Definition Language (DDL): used to define files, records, and
attributes
Data Manipulation Language (DML, Query Language): Simple
Query formulation (able to express only about 60% of typical queries)
Algorithmic Language (AL): APIs for COBOL, FORTRAN, PL1, …
(used to formulate more complex queries)
Advantages:
Data Sharing
Atomic Transaction and Automatic Recovery (70s): page-level logical
logging
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Concurrent Access (70s): two-phase locking, time stamping
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Some security
Remaining Problems:
Data Redundancy and Inconsistency
Limited expression power of DML
Security violations are possible
New Data Models:
Relational model (E.F. Codd 1970)
Network model (CODASYL Data Task Group, 1972)
ER model (Chen, 1976)
By the end of 70's, 4000 GFMSs installed
• Late 70s - 80's: Third Generation DBS
• Late 80's - Present: Fourth Generation DBS: Based on the
third generation but equipped with the extended featured for:
Distributed, Client/Server DBS
Parallel DBS
Object-Oriented DBS/Object-Relational DBS
Spatial Databases for storage and retrieval of multimedia
data, image data, scientific data analysis, geographic
information, biomedical data, multi-attribute indexing…
Data warehousing, …
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3. Three Levels of Abstraction (Three Level Schema)
Program1
View1
Program2
View2
View3
Program3
View4
View Level
Schema
Logical Level Schema
Physical Level Database Schema
Figure 4 Three Level Schema
• Provide the environment to solve the data redundancy and
inconsistency problem
• Data Independence
Physical Data Independence: The ability to modify the
physical schema without causing application programs to be
rewritten.
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Logical Data Independence: The ability to modify the
logical schema without causing the application programs to
be rewritten
More Security: View Level Schema
4. Data Models
• Object-Based Logical Models: Entity-Relationship (ER) model,
Object-Oriented model, etc.
• Record-Based Logical Models: Relational model, Network model,
and Hierarchical model
• Usually, a DBMS is based on a certain logical data model:
Relational DBMS, Network DBMS, Hierarchical DBMS, ObjectOriented DBMS.
• Modern DBMS are based on Relational Model + Object Model
(a.k.a. the Object-Relational Model)
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5. General Database System Structure
Library
(API)
DML
translator
DDL
interpreter
Query Optimizer
Query Evaluation Engine
Transaction Manager
Buffer Manager
File Manager
Indices
Data Dictionary
Data Files
Statistical data
Figure 5 An Example of DBS
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