Transcript Lecture 1 : Introduction to DBMS

CENG 352
Database Management Systems
Instructor: Nihan Kesim Çiçekli
email: [email protected]
URL: http://www.ceng.metu.edu.tr/~nihan
CENG 352
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Instructor: Nihan Kesim Çiçekli
Office: A308
Email: [email protected]
Lecture Hours: Mon. 14:40,15:40 (BMB4);
Thu. 13:40 (BMB2)
• Office Hours: Fri. 10:40-11:30
• Course Web page:
http://www.ceng.metu.edu.tr/~semra/nli/ceng352
• Teaching Assistant:
Semra Doğandağ ([email protected])
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Text Books and References
1. Raghu Ramakrishnan, Database Management
Systems, McGraw Hill, 3rd edition, 2003 (text
book).
2. R. Elmasri, S.B. Navathe, Fundamentals of
Database Systems, 4th edition, Addison-Wesley,
2004.
3. A. Silberschatz, H.F. Korth, S. Sudarshan,
Database System Concepts, McGraw Hill, 4th
edition, 2002.
4. H. Garcia-Molina, J. D. Ullman, J. Widom,
Database Systems The Complete Book, Prentice
Hall, 2002.
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Grading
Written assignments
Project
Midterm Exam
Final
15%
20%
30%
35%
Exam Date:
Midterm Exam: 2nd week of April.
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Grading Policies
• Policy on missed midterm:
– no make-up exam
• Lateness policy:
– Late assignments are penalized up to 10% per
day.
• All assignments are to be your own work.
Projects in groups of two.
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Course Outline
• The Relational Data Model, Relational Algebra
and Calculus, SQL
• Query Evaluation and Optimization
• Relational Database Design and Tuning
• Transaction Management, Concurrency Control
and Crash Recovery
• Database Security and Authorization
• Parallel and Distributed Databases
• Object-Database Systems
• Information Retrieval and XML Data
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What is a Database Management System?
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A Database Management System (DBMS) is a
software package designed to store and manage
databases:
1. Manages very large amounts of data.
2. Supports efficient access to very large amounts of data.
3. Supports concurrent access to very large amounts of data.
•
Example: bank and its ATM machines.
4. Supports secure, atomic access to very large amounts of
data.
•
Contrast two people editing the same UNIX file – last to write
“wins” – with the problem if two people deduct money from the
same account via ATM machines at the same time – new balance is
wrong whichever writes last.
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Example: Online Bookseller
• Data = information on books (including categories,
bestsellers, etc.), customers, pending orders, order
histories, trends and preferences, etc.
– Massive: many gigabytes at a minimum for mediumsize bookseller, more if keep all order histories over all
time, even more if keep images of book covers and
sample pages
=> Far too big for memory
– Persistent: data outlives programs that operate on it
– Multi-user: many people/programs accessing same
database, or even same data, simultaneously
=> Need careful controls
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Files vs. DBMS
• Application must stage large datasets between
main memory and secondary storage (e.g.,
buffering, page-oriented access, 32-bit addressing,
etc.)
• Special code for different queries
• Must protect data from inconsistency due to
multiple concurrent users
• Crash recovery
• Security and access control
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What is a Relational Database?
• Based on the relational model (tables):
acct #
12345
34567
…
name
Sally
Sue
…
balance
1000.21
285.48
…
• Today used in most DBMS's.
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The DBMS Marketplace
• Relational DBMS companies – Oracle, Sybase – are among the
largest software companies in the world.
• IBM offers its relational DB2 system. With IMS, a nonrelational
system, IBM is by some accounts the largest DBMS vendor in the
world.
• Microsoft offers SQL-Server, plus Microsoft Access for the cheap
DBMS on the desktop, answered by “lite” systems from other
competitors.
• Relational companies also challenged by “object-oriented DB”
companies.
• But countered with “object-relational” systems, which retain the
relational core while allowing type extension as in OO systems.
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Three Aspects to Studying DBMS's
1. Modeling and design of databases.
– Allows exploration of issues before committing to an
implementation.
2. Programming: queries and DB operations like
update.
– SQL = “intergalactic dataspeak.”
3. DBMS implementation.
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Query Languages
Employee
Name
Department
Dept
Dept
Manager
SQL
SELECT Manager
FROM Employee, Department
WHERE Employee.name = "Clark Kent”
AND Employee.Dept = Department.Dept
Query Language
Data definition language (DDL) ~ like type defs in C or Pascal
Data Manipulation Language (DML)
Query (SELECT)
UPDATE < relation name >
SET <attribute> = < new-value>
WHERE <condition>
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Host Languages
C, C++, Java, Lisp, COBOL
Application prog.
Calls to
DB
DBMS
Local Vars
(Memory)
(Storage)
• Host language is completely general (Turing complete)
• Query language—less general "non procedural" and
optimizable
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Relational Model
Relational model is good for:
• Large amounts of data —> simple operations
• Navigate among small number of relations
Difficult Applications for relational model:
• VLSI Design (CAD in general)
• CASE
• Graphical Data
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Other Models
Where number of "relations" is large, relationships are complex
•Object Data Model
•Logic Data Model
OBJECT DATA MODEL
1. Complex Objects – Nested Structure (pointers or
references)
2. Encapsulation, set of Methods/Access functions
3. Object Identity
4. Inheritance – Defining new classes like old classes
Object model: usually find objects via explicit navigation
Also query language in some systems
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Other Models
LOGIC (Horn Clause) DATA MODEL
• Prolog, Datalog:
if A1 and A2 then B
B:- A1 and A2
• Functions s(5) = 6 (successor)
• Predicates with Arguments: sum(X,Y,Z)  X + Y = Z
sum(X,0,X) means X + 0 = X (always true for all X)
sum(X,s(Y),s(Z)):-sum(X,Y,Z) means X+(Y+1) = (Z+1) if X
+Y=Z
• More powerful than relational
Can Compute Transitive Closure
edge(X,Y).
path(X,Y) :- edge(X,Y).
path(X,Z) :- path(X,Y) & edge(Y,Z).
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Data Models
60’s
Hierarchical
Network
70's
80's
Choice for most new
applications
Relational
90’s
Object Bases
Knowledge Bases
now
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Why Use a DBMS?
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•
•
•
•
Data independence and efficient access.
Reduced application development time.
Data integrity and security.
Uniform data administration.
Concurrent access, recovery from crashes.
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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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Levels of Abstraction
• Many views, single
View 1 View 2 View 3
conceptual (logical) schema
and physical schema.
Conceptual Schema
–
–
–
Views describe how users see
the data.
Conceptual schema defines
logical structure
Physical schema describes the
files and indexes used.
Physical Schema
* Schemas are defined using DDL; data is modified/queried using DML.
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Concurrency Control
• Concurrent execution of user programs is essential
for good DBMS performance.
–
Because disk accesses are frequent, and relatively slow, it
is important to keep the cpu humming by working on
several user programs concurrently.
• Interleaving actions of different user programs can
lead to inconsistency: e.g., check is cleared while
account balance is being computed.
• DBMS ensures such problems don’t arise: users can
pretend they are using a single-user system.
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Transaction: An Execution of a DB Program
• Key concept is transaction, which is an atomic
sequence of database actions (reads/writes).
• Each transaction, executed completely, must leave the
DB in a consistent state if DB is consistent when the
transaction begins.
–
–
–
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. (e.g., it does not understand how the
interest on a bank account is computed).
Thus, ensuring that a transaction (run alone) preserves
consistency is ultimately the user’s responsibility!
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Scheduling Concurrent Transactions
• DBMS ensures that execution of {T1, ... , Tn} is
equivalent to some serial execution T1’ ... Tn’.
–
–
–
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.
What if Tj already has a lock on Y and Ti later requests a lock
on Y? (Deadlock!) Ti or Tj is aborted and restarted!
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Ensuring Atomicity
• DBMS ensures atomicity (all-or-nothing property) even
if system crashes in the middle of a Xact.
• Idea: Keep a log (history) of all actions carried out by
the DBMS while executing a set of Xacts:
–
–
Before a change is made to the database, the corresponding log
entry is forced to a safe location. (WAL protocol; OS support
for this is often inadequate.)
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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The Log
• The following actions are recorded in the log:
–
Ti writes an object: The old value and the new value.
• Log record must go to disk before the changed page!
–
Ti commits/aborts: A log record indicating this action.
• Log records chained together by Xact id, so it’s
easy to undo a specific Xact (e.g., to resolve a
deadlock).
• Log is often duplexed and archived on “stable”
storage.
• All log related activities (and in fact, all CC related
activities such as lock/unlock, dealing with
deadlocks etc.) are handled transparently by the
DBMS.
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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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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.
These layers
must consider
concurrency
control and
recovery
Query Optimization
and Execution
Relational Operators
Files and Access Methods
Buffer Management
Disk Space Management
DB
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Summary
• 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.
• DBAs hold responsible jobs
and
are well-paid! 
• DBMS R&D is one of the broadest,
most exciting areas in CS.
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