Transcript Introduction to Database Systems

CS 4332
Introduction to Database Systems
Byron Gao
Logistics
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Instructor: Byron Gao
 Page: http://cs.txstate.edu/~jg66/
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Course page
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Textbook
workload
Grading: nice curve, but won’t please
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History (usually you get what you expect)
Weighting
Late policy
Academic honesty
TRACS: https://tracs.txstate.edu/portal
Objective
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Provide in-depth introduction to database
management systems, emphasizing how to design a
database and use a DBMS effectively
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Relational Database Management Systems (RDBMS)
Relational algebra
Structured Query Language (SQL)
Database design
 Web application
 Data warehousing and data mining
 XML data and unstructured data management
Chapter 1
Overview of Database Systems
What Is a DBMS?
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Science paradigms: empirical, theoretical,
computational, data-intensive
 Data deluge, information explosion, information overload
 Databases (structured), data mining, information retrieval
 DBA … data scientists (more data mining, analytics …)
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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 CS 4332)
A Database Management System (DBMS) is a software
package designed to store and manage databases
Example: use of structured data
mysql -u jg66 -h mysql.cs.txstate.edu –p
show databases;
use jg66;
show tables;
select * from Sailors;
select * from Boats;
select * from Reserves;
Find the names of sailors who’ve reserved a red boat?
SELECT S.sname
FROM Sailors S, Boats B, Reserves R
WHERE S.sid=R.sid and B.bid=R.bid and B.color="red";
More examples
Find the colors of boats reserved by Lubber?
SELECT B.color
FROM Sailors S, Boats B, Reserves R
WHERE S.sid=R.sid and B.bid=R.bid and S.sname="Lubber";
Find sid’s of sailors who’ve reserved red but not green boats?
Find the age of the youngest sailor with age 18, for each rating with at least 2
such sailors?
Find those ratings for which the average age is the minimum over all
ratings?
Historical Perspective
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Early 60’s: network data model
 Charles Bachman, 1973 first Turing award
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Late 60’s: hierarchical data model, IBM
70: relational data model
 Edgar Codd, IBM Research, 1981 Turing award
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80’s:
 SQL, IBM System-R;
 Transactions, concurrent execution of database programs, Jim Gary,
1999 Turing award
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Late 80’s and 90’s:
 Storing images and text, more complex queries, data warehouses
 Internet era, files -> DBMS
 Decision support, data mining
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Recently, managing unstructured data
DB2, Oracle, Sql Server, Informix ... Ingres, PostgreSql … MySql
Files vs. DBMS
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OS inadequate
 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?
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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Reasons not to use DBMS
Why Study Databases??
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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
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OS, languages, theory, AI, multimedia, logic
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 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
Levels of Abstraction
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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
Example: University Database
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Conceptual (logical) 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
External Schema (View):
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course_info(cid:string,enrollment:integer)
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
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 One of the most important benefits of using a DBMS!
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 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!
Scheduling Concurrent Transactions
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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!
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!)
The Log
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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!
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
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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