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Database Management Systems
Chapter 1
Overview of DBMSs
Professor: Iluju Kiringa
[email protected]
http://www.site.uottawa.ca/~kiringa
SITE 5072
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Objectives

Definition of a DBMS
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Data models
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Definition -- Data abstraction -- Data independence
Main topics of study
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What is a DBMS? -- Why use and study DBMSs?
Why is a DBMS different than a file system?
File management -- Query and transaction processing
Data independence --Architecture of DBMSs
Miscellaneous
 Actors -- History
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What Is a DBMS?
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Definition
 A database is a very large, integrated collection of
data.
 A Database Management System (DBMS) is a software
package designed to store and manage databases.
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Models real-world organization / enterprise.
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Entities (e.g., students, courses, faculty, and
classrooms)
Relationships (e.g., Sue is enrolled in CSI3317; Iluju
teaches CSI3317, CSI3317 is taught in TBT070)
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Drawbacks of Files Systems
Each application must move large datasets
between main memory and secondary storage
(must deal with, e.g., buffering, page-oriented
access, etc).
 Each application must deal with some method
of identifying all data items in case the
available addressing mode is not sufficient
(e.g., 32-bit addressing cannot directly access
more than 4GB).
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Drawbacks of Files Systems (Cont’d)
Need special code for different queries (i.e. all
query codes are ad hoc).
 Must protect data from inconsistency due to
multiple concurrent users changing it.
 Must ensure consistent crash recovery.
 Must provide more security and access control
than the password mechanism offered by
operating systems.
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Why Use a DBMS?
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Data independence: application don’t see details of
data representation and storage.
Efficient access: use of very sophisticated data storage
and access methods.
Reduced application development time:
functionalities of a DBMS need not be duplicated.
Data integrity and security: enforcing integrity
constraints and access control.
Uniform data administration: experienced users
administer data that is used by inexperienced ones.
Concurrent access, recovery from crashes: users
access data without thinking of whoever else uses it.
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Why Study Databases??
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Datasets increasing in diversity and volume
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Today’s HUGE amounts of data need to be efficiently
managed – Some applications involve PBs of data
Digital libraries, interactive video, Human Genome project,
EOS project, Mars exploration projects, …
... need for DBMS exploding
Efficient management of such a diverse amount of
data involves research in many fundamental issues
DBMS encompasses most of CS

OS, languages, theory, AI, multimedia, logic, etc.
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Data Models
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A data model is a collection of high-level
constructs for describing data.
A schema is a description of a particular collection
of data, using a given data model.
An instance of a schema is a sample set of data
organized using a given schema.
The relational model is the most widely used
model today.
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Main concept: relation instance (relation), i.e. a
table with rows and columns.
Every relation has a relation schema (schema),
which describes the name and columns, or fields.
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Levels of Abstraction
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Data schemas are given at
3 levels of abstractions.
Many views, single
conceptual (logical) schema
and physical schema.
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Views (or external schemas)
describe how users see the
data.
Conceptual schema defines
logical structure
Physical schema describes
the files and indexes used.
View 1
View 2
View 3
Conceptual Schema
Physical Schema
* Schemas are defined using a data definition language (DDL).
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Example: Enterprise Database
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Conceptual schema:
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Physical schema:
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Emp(eid: integer, ename: string, age: integer, sal: real)
Works_in(eid: integer, did: integer, rating: real)
Dept(did: integer, budget: real, mgr: integer)
Relations stored as ordered files.
Index on first column of Emp, and Dept; index on
all columns of Works_in, etc.
External Schema (View):
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Dept_count(did: integer, count: integer)
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Example: University Database
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Conceptual schema:
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Physical schema:
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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)
Relations stored as unordered files.
Index on first column of Students, and Courses; index
on two first columns of Enrolled, etc.
External Schema (View):
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Course_info(cid: string, enrollment: integer)
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Data Independence
Applications insulated from how data is
structured and stored.
 Each level of abstraction protected from
changes in the structure of the level below it.
 Logical data independence: Protection from
changes in logical structure of data.
 Physical data independence: Protection from
changes in physical structure of data.
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Querying Data
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A query is a question involving the stored data, e.g.,
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What is the name of the professor who teaches CSI3317?
Which percentage of students got an A+ in CSI3317?
A query language is a special purpose language in
which queries can be posed against databases.
Data is modified/queried using a data manipulation
language (DML). So a query language is a subset of a
DML
The relational model supports 2 query languages:
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Relational calculus: logic-based query language
Relational algebra: based on a set of operators for
manipulating relations.
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Transactions
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A transaction is an atomic sequence of database actions
(reads/writes) corresponding to the execution of a
DB transaction program.
Each transaction, executed completely, must leave
the DB in a consistent state if DB is consistent when
the transaction begins.
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Users can specify some simple integrity constraints on the
data, and the DBMS will enforce these constraints.
Beyond this, the DBMS does not really understand the
semantics of the data. Thus, ensuring that a transaction
preserves consistency is ultimately the user’s
responsibility!
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Transactions: Concurrency Control
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Concurrent execution of user programs is
essential for good DBMS performance.
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Because disk accesses are frequent, and relatively slow,
it is important to keep the cpu working by processing
several user programs concurrently.
Interleaving actions of different user programs
can lead to inconsistency: e.g., depositing money
while account balance is being computed may
lead to lost of money if not done properly.
 DBMS ensures such problems don’t arise: users
can pretend they are using a single-user system.
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Scheduling Concurrent Transactions
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A DBMS ensures that execution of {T1, ... , Tn} is
equivalent to some serial execution of T1, ..., Tn.
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Before reading/writing an object, a transaction requests
a lock on the object, and waits till the DBMS gives it the
lock. All locks are released at the end of the transaction.
(Strict 2PL locking protocol.)
Idea: If an action of Ti (say, writing X) affects Tj (which
perhaps reads X), one of them, say Ti, will obtain the
lock on X first and Tj is forced to wait until Ti completes;
this effectively orders the transactions.
If Tj already has a lock on Y and Ti later requests a lock
on Y, there is Deadlock! Ti or Tj is aborted and restarted!
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Transactions: Crash Recovery
A DBMS ensures atomicity (all-or-nothing
property) even if there is a crash in the middle of a
transaction.
 Idea: Keep a log (history) of all actions carried out
by the DBMS while executing a set of transactions:
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Before a change is made to the database, the
corresponding log entry is forced to a safe location.
(WAL protocol)
After a crash, the effects of partially executed
transactions are undone using the log. (Thanks to WAL, if
log entry wasn’t saved before the crash, corresponding
change was not applied to database!)
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People Involved with Databases
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End users: just use an interface to a DBMS!
DBMS vendors: IBM, Oracle, Informix, Microsoft, etc
DBMS researchers and implementers: invent new
theories and algorithms for, and write code of DBMSs
DB application programmers: write C/C++/Java/…
programs that interact with DBMSs
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E.g. smart webmasters, CSI3317 students, etc
Database administrator (DBA)
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Designs logical /physical schemas
Handles security and authorization
Data availability, crash recovery
Database tuning as needs evolve
DBAs must understand how a DBMS works!
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These layers
must consider
concurrency
control and
recovery
Structure of a DBMS
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A typical DBMS has a
Query Optimization
layered architecture.
and Execution
The figure does not
Relational Operators
show the concurrency
Files and Access Methods
control and recovery
components.
Buffer Management
This is one of several
Disk Space Management
possible architectures;
each system has its own
variations.
DB
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Historical Perspective
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Early 1960s: GE designed 1st general-purpose DBMS,
based on the network data model.
Late 1960s: IBM developed IMS, based on the
hierarchical data model.
Early 1970s: E. Codd introduced the relational model
to solve problems related to the previous models.
Late 1970s and early 1980s: work on transaction
processing, mainly by Jim Gray and P. Bernstein,
All the 1980s: use of relational DBs became standard
practice in large corporations, many vendors entered
the market, standardization efforts on SQL, the query
language for relational DBs, etc.
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Historical Perspective (Cont’d)
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Late 1980’s and early 1990s: richer data models (objectoriented, object-relational, etc), and more expressive
query languages (Datalog, nested relations, etc) are
introduced.
Late 1990s: major vendors extend relational DBMSs
towards new data types (images, text, multimedia
content) and queries based hereon.
All along the way:
 Scientific recognition: Turing Awards to DB
researchers E. Codd and J. Gray
 Birth of a huge, multibillion industry (IBM, Oracle …)
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Summary
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DBMS 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
DB researchers, implementers, and administrators
hold responsible jobs and are well-paid
DBMS R&D is a very broad and exciting area of CS
Database companies form a multibillion industry
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