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Introduction to Database Systems

Chapter 1

Instructor: Mirsad Hadzikadic

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http://www.sigmod.org/record/issues/0606/index.html

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History

60s C. Bachman GE, network data model, CODASYL Late 60s IBM IMS, hierarchical data model, SABRE, AA-IBM 70 Edgar Codd, IBM, relational model 80s SQL, IBM, System R project, concurrent

transactions management, J. Gray Late 80-90s DB2, Oracle, Informix, Sybase ERP, MRP Baan, Oracle, PeopleSoft, SAP, Siebel

Common tasks – inventory, HR, financial analysis 90s DW, Internet Object-oriented, object-relational DBs Turing award and Turing test?

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Why Study Databases?

Data everywhere Shift from computation to information

– At the “low end:” scramble to Web space– At the “high end:” scientific applications

Datasets increasing in diversity and volume – Digital libraries, interactive video, Human Genome

project, EOS project – ... need for DBMS exploding

DBMS encompasses most of CS– OS, languages, theory, “A”I, multimedia, logic

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What Is a DBMS?

Files vs. Database A very large, integrated collection of data. Models real-world enterprise

– Entities (e.g., students, courses)– Relationships (e.g., Madonna is taking ITCS

6160) A Database Management System (DBMS)

is a software package designed to maintain and utilize databases

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Why Use a DBMS?

Data independence and efficient access Data integrity and security Uniform data administration Concurrent access, recovery from

crashes Reduced application development time When not to use a DB?

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Data Models A data model is a collection of concepts for

describing data A schema is a description of a particular

collection of data, using the given data model The relational model of data is the most

widely used model today– Main concept: relation, basically a table with rows

and columns– Every relation has a schema, which describes the

columns, or fields

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Levels of Abstraction

Many (external) views, single conceptual (logical) schema and physical schema.– Views describe how

users see the data

– Conceptual schema defines logical structure

– Physical schema describes the files and indexes used Views and Schemas are defined using DDL; data is modified/queried using DML

Physical Schema

Conceptual Schema

View 1 View 2 View 3

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Example: University Database

Conceptual schema: – Students(sid: string, name: string, login: string,

age: integer, gpa:real)– Courses(cid: string, cname:string, credits:integer) – Enrolled(sid:string, cid:string, grade:string)

Physical schema:– Relations stored as unordered files – Index on first column of Students

External Schema (View): – Course_info(cid:string, enrollment:integer)

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Data Independence

Applications insulated from how data is structured and stored

Logical data independence: Protection from changes in logical structure of data

Physical data independence: Protection from changes in physical structure of data

One of the most important benefits of using a DBMS!

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Structure of a DBMS

A typical DBMS has a layered architecture

The figure does not show the concurrency control and recovery components

This is one of several possible architectures; each system has its own variations

Query Optimizationand Execution

Relational Operators

Files and Access Methods

Buffer Management

Disk Space Management

DB

These layersmust considerconcurrencycontrol andrecovery

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Transaction Management: ACID properties

AAtomicity: All actions in the Xact happen, or none happen

CConsistency: If each Xact is consistent, and the DB starts consistent, it ends up consistent

IIsolation: Execution of one Xact is isolated from that of other Xacts

DDurability: If a Xact commits, its effects persist

The Recovery Manager guarantees Atomicity & Durability

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Motivation of concurrency control

Consistency Isolation Example

– Two parallel transactions T1 and T2– Serial execution– Execution with interleaving actions– Example

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Motivation of recovery management

Atomicity: – Transactions may abort (“Rollback”)

Durability:– What if DBMS stops running? (Causes?)

crash! Desired Behavior after

system restarts:– T1, T2 & T3 should be

durable– T4 & T5 should be

aborted (effects not seen)

T1T2T3T4T5

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Databases make these folks happy ...

End users and DBMS vendors DB application programmers

– E.g. smart webmasters Database administrator (DBA)

– Designs logical /physical schemas– Handles security and authorization– Data availability, crash recovery – Database tuning as needs evolve

Must understand how a DBMS works!

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Summary DBMS is used to maintain, query large datasets Benefits include recovery from system crashes,

concurrent access, quick application development, data integrity and security

Levels of abstraction give data independence A DBMS typically has a layered architecture DBAs hold responsible jobs and are

well-paid! DBMS R&D is one of the broadest,

most exciting areas in CS