Download Database Management Systems Chapter 1 What Is a DBMS?

Database Management Systems
Chapter 1
Overview of Database Systems
Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke
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What Is a DBMS?
A database is a collection of data.
Models real-world enterprise, e.g. university.
•
•
Entities (e.g., students, courses)
Relationships (e.g., Tom is taking CISC6325)
A Database Management System (DBMS) is a
software package designed to store and
manage databases.
Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke
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Why we need DBMS
Huge amount of data is available.
Users need tools that simplify the tasks of
managing the data and extracting useful
information in a timely fashion.
Otherwise, data is stored in files and write
application-specific code to manage it.
Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke
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Files vs. DBMS
Drawbacks of Files
Application must stage large datasets between
main memory and secondary storage.
Writing special code to identify all data items.
Writing special code for different queries – users’
questions about data
Must protect data from inconsistency due to
multiple concurrent users
Crash recovery
Security and access control
Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke
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Why Use a DBMS?
Data independence.
DBMS provides an abstract view of data and hides the
details of data representation and storage.
Efficient data access.
Sophisticated techniques to store and retrieve data
efficiently.
Data integrity and security.
Enforce integrity constraints.
Perform access controls.
Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke
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Why Use a DBMS?
Uniform data administration.
Organizing the data representation to minimize redundancy.
Tuning the storage for efficient retrieval.
Concurrent access, recovery from crashes.
Users are not aware of others while accessing DBMS.
Protects users from the system failures.
Reduced application development time.
Well designed and tested functions and user interfaces.
Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke
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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, AI, multimedia, logic
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Data Models
A data model is a collection of concepts for
describing data. Users of DBMS define the
data stored in terms of a data model.
High-level data description that hide low-level
storage details.
Most popular: relational data model.
A semantic data model is a more abstract, highlevel data model that makes it easier for users
to describe data in an enterprise.
Example: the entity-relationship (ER) model
ER model => The Relational Data Model => DBMS
Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke
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The Relational Model
A schema is a description of a particular
collection of data in terms of a data model.
Main concept of the Relational Model :
A relation is a set of records.
Basically a table with rows and columns.
Every relation has a schema, which describes the
columns, or fields.
Students (sid:string, name:string, login:string, gpa:real)
sid
name
login
gpa
53666
Jones
jones@cs
3.4
53832
Guldu
guldu@music
2.0
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Level of Abstraction in a DBMS
View 1
View 2
View 3
Conceptual Schema
Physical Schema
Disk
Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke
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Levels of Abstraction in a DBMS
Many views (external schema), 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.
A Data Definition Language (DDL) is used to
define the external and conceptual schemas.
SQL
Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke
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Example: University Database
Conceptual schema (logical schema) describes
all relations that are stored in the database.
Entities
•
•
Students(sid: string, name: string, login: string,
age: integer, gpa:real)
Courses(cid: string, cname:string, credits:integer)
Relationships
•
Enrolled(sid:string, cid:string, grade:string)
Conceptual database design
Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke
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Example: University Database
Physical schema specifies additional storage
details.
Relations stored as unordered files.
Index (auxiliary data structure to speed update
retrieval operations) on first column or other
column of relations.
Physical database design
Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke
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Example: University Database
External Schema allows data access to be
customized (and authorized) at the level of
individual users or groups of users.
Each external schema contains a set of Views.
A view is conceptually a relation, which is not
stored in the DBMS.
Example: Course_info (cid:string,enrollment:integer)
A DBMS has one conceptual schema, one
physical schema and many external schemas.
Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke
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Data Independence *
Applications insulated from how data is
structured and stored.
Logical data independence : By generating
views, changes in logical structure of data
(conceptual schema) is hidden.
Physical data independence : The conceptual
schema insulates users from changes in
physical structure of data.
* One of the most important benefits of using a DBMS!
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Queries in a DBMS
Questions involving the data stored in a
DBMS are called Queries.
What is the name of the student with ID 123?
How many students are enrolled in CS6325?
A DBMS provides a specialized language,
called the query language, in which queries
can be posed.
Formal Query Languages:
• Relational calculus
• Relational algebra.
Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke
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Data Manipulation Language
A DBMS enables users to create, modify, and
query data through a Data Manipulation
Language (DML).
The query language is part of DML.
SQL
* 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.
Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke
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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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Concurrent Execution of Transactions
It is important that a DBMS guarantees safe
concurrent data access.
Multiple users could access data as if he is the
only user.
DBMS ensures that execution of {T1, ... , Tn} is
equivalent to some serial execution T1’ ... Tn’.
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Scheduling Concurrent Transactions
A lock is a mechanism used to control access to database
objects.
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!
Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke
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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!)
Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke
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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.
Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke
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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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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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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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