Transcript Database Management System

Why do you learn database??
Chapter 0
Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke
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DB = IT Core Platform in the 21st
Century?
ERP/CRM/SEM/BSC
DW/OLAP/Data Mining
Web Log Analysis
GIS/XML/Mobile
Bio-informatics
....
Ubiquitous Databases
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Why DB Tuning in 21st Century?
DB performance influences all aspects of your
business
 DB performance management is one of the most
important success factors in almost every IT
projects
 “미래 웹경쟁력은 데이터처리능력에”(Scott
McNealy, Sun CEO, 전자신문)
DB tuning is not a technical issue, but a
business issue!
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DB Performance Impact: Real Story
After tuning SQLs in a Shopping mall
application programs
 30% (50억) increase in Sales
When an index is dropped in an Internet
auction site
 Reduction in benefit: 2억/day
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Databases: Bigger, Complexer, Faster
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More data - doubling every 9 month
More users - from CEO to sales representatives
More complex query - e.g. OLAP/data mining query
Faster responses
e.g. web response time impacts revenue!
e.g. real time personalization
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Disk
Sales
Moore's
Law
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Petabytes
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Year
Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke
 Data growth vs.
computer
speedup
* Moore’s Law -- #
of transistors/chip
doubles every 18
months (1965)
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Real Customer Story
Raw Data (excl. indexes, etc.)
Users
Queries / Day
Percent Queries < 5 mins
Direct SQL Access
User Schedule/Publish Reports
2000 2001
~1 TB 2.3 TB
330
512
630 1,000
63%
77%
No
Yes
No
Yes
2002
9 TB
800
4,300
80%
Yes
Yes
2003
13 TB
800
6,000
80%
Yes
Yes
In just three years:
- 13x growth in raw data
- 10x growth in number of queries
- ~3x growth in number of users
- additional lines of business supported
- increasing numbers of partners supported
...with better performance!
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Database Management Systems
Chapter 1
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Sophisticated users, application
programmers, DB administrators
Unsophisticated users (customers, travel agents, etc.)
Web Forms
Application Front Ends
SQL Interface
SQL COMMANDS
shows command flow
Plan Executor
Parser
Operator Evaluator
Optimizer
shows interaction
Query
Evaluation
Engine
Files and Access Methods
Transaction
Manager
Recovery
Buffer Manager
Lock
Manager
Manager
Disk Space Manager
Concurrency
Control
DBMS
shows references
Index Files
System Catalog
Data Files
DATABASE
Figure 1.3 Architecture of a DBMS
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What Is a DBMS?
A very large, integrated collection of data.
Models real-world enterprise.
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Entities (e.g., students, courses)
Relationships (e.g., Madonna is taking CS564)
A Database Management System (DBMS) is a
software package designed to store and
manage databases.
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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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Why Use a DBMS?
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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Why Study Databases??
?
Shift from computation to information
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at the “low end”: scramble to webspace (a mess!)
at the “high end”: scientific applications
Datasets increasing in diversity and volume.
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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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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 a given data
model.
The relational model of data is the most widely
used model today.
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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 views, single
conceptual (logical) schema
and physical schema.
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Views 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 DDL; data is modified/queried using DML.
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Example: University Database
Conceptual 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)
Physical schema:
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Relations stored as unordered files.
Index on first column of Students.
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.
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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Concurrency Control
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 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.
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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. (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’.
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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.
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:
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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:
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Ti writes an object: the old value and the new value.
• Log record must go to disk before the changed page!
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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
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E.g. smart webmasters
Database administrator (DBA)
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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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These layers
must consider
concurrency
control and
recovery
Structure of a DBMS
 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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Sophisticated users, application
programmers, DB administrators
Unsophisticated users (customers, travel agents, etc.)
Web Forms
Application Front Ends
SQL Interface
SQL COMMANDS
shows command flow
Plan Executor
Parser
Operator Evaluator
Optimizer
shows interaction
Query
Evaluation
Engine
Files and Access Methods
Transaction
Manager
Recovery
Buffer Manager
Lock
Manager
Manager
Disk Space Manager
Concurrency
Control
DBMS
shows references
Index Files
System Catalog
Data Files
DATABASE
Figure 1.3 Architecture of a DBMS
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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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