ch1_intro_HH_upd lectures othe be hand.ppt

Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke
Database Management Systems
Chapter 1
Hazem Hajj
Presentations based on Course Material

Content
 What is a DBMS? Why use it?
 Describing and Storing data in a DBMS
 Relational Model
 Levels of abstraction
 Queries in DBMS
 Transaction Management
 Concurrent execution
 Incomplete transaction and system crash
 Structure of a DBMS

DBMS – A Historical Perspective
 1960s:
 Data collection, database creation, IDS (early 60s Integrated data store) and network DBMS.
ACM Turing Award for Charles Bachman (GE).
 1970s:
 Relational data model, relational DBMS implementation. ACM Turing Award to Edgard
Codd (IBM)
 1980s:
 RDBMS, advanced data models (extended-relational, OO, deductive, etc.)
 Application-oriented DBMS (spatial, scientific, engineering, etc.)
 SQL – standard query Language
 1990s:
 Data mining, data warehousing, multimedia databases, and Web databases
 2000s
 Stream data management and mining
 Data mining and its applications
 Major DB Projects: NASA’s Earth Observation System and Human Genome Mapping Project
 Web technology (XML, data integration) and global information systems

Why Study Databases??
 Shift from computation to information
 at the “low end”: scramble to webspace (a mess!)
 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, multimedia, logic
?

Why Study DBMS – For data mining
 Data mining—”Getting
the golden data nuggets
out of data tombs”
Data Cleaning
Data Integration
Databases
Data
Warehouse
Task-relevant Data
Selection
Data Mining
Pattern Evaluation

What Is a DBMS?
 A very large, integrated collection of data.
 Models real-world enterprise (one or more
related organizations, e.g. university)
 Entities (e.g., students, courses)
 Relationships (e.g., Obama is taking EECE433)
 A Database Management System (DBMS) is a
software package designed to store and
manage databases.

Files vs. DBMS
 Using Files, the programmer would need to
handle:
 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

Why Use a DBMS?
 A DBMS would address all previous issues. It would
provide:
 Data independence (No need to know how the data is
stored in a class/data structure)
 Efficient access (e.g. use of indexing and hashing)
 Reduced application development time
 Data integrity (e.g. integrity constraints- no grade above
100) and security (need to know access)
 Uniform data administration (one for all users)
 Concurrent access (multiple users), recovery from crashes.
Are there cases where we would not want to use a DBMS?

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Describing & Storing Data in a
DBMS - Data Models
 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.
• Example: Table with: SID | NAME | LOGIN |
AGE
 Semantic data model – higher level of
abstraction – Use ER model

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Lec2

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DBMS - Levels of Abstraction
 Three Levels of Abstraction
 Many 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.
NOTE Schemas & views 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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DBMS - 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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Content
 Queries in DBMS

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Queries in a DBMS
 What is the name of the student with student
ID = 2345?
 How many students are enrolled in
EECE433?
 Use SQL

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Transaction Management -
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 Management - Transaction:
An Execution of a DB Program
 Key concept is transaction, which is an atomic sequence of
database actions (reads/writes). Atomic => All happens
or none.
 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!

2
Transaction Management - 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: Two-Phase 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!

2
Transaction Management - Ensuring Atomicity &
Dealing with Incomplete transaction, e.g crash
 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: Write Ahead Log 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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Transaction Management - 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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Content
 Queries in DBMS

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Structure of a DBMS
 A typical DBMS has a
layered architecture.
 This is one of several
possible architectures;
each system has its own
variations.
Query Optimization
and Execution
Relational Operators
Files and Access Methods
Buffer Management
Disk Space Management
DB
concurre
ncy
control
and
recovery
SQL Commands

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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 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.

ch1_intro_HH_upd lectures othe be hand.ppt

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