Syllabus

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Syllabus

• Instructor???(IM309)
• TA???(IM300)
• Textbook (I) Database Management Systems
• by Ramakrishnan Gehrke
• (II) Data Mining-Concepts
  and
• Techniques, by
  Jiawei Han
• and Micheline
  Kamber
• Grading policy, Mid-term Final Exam.

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Unit I Introduction to Database Systems
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Contents of this Unit

• Basic concepts
• file systems DBMSs
• data modeling
• concurrent, fault-tolerant data management
• DBMS architecture
• An introduction to the Relational Model SQL

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What Is a DBMS?

• A very large, integrated collection of data.
• Models real-world enterprise.
• Entities (e.g., students, courses)
• Relationships (e.g., Janet Reno is taking CS186)
• A Database Management System (DBMS) is a software
  package designed to store and manage databases.

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Database Management Systems

• What more could we want than a file system?
• Simple, efficient ad hoc1 queries
• concurrency control
• recovery
• benefits of good data modeling

1ad hoc formed or used for specific or immediate
problems or needs
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Functionality of a DBMS

• Persistent storage management
• Transaction management
• Resiliency recovery from crashes.
• Separation between logical and physical views of
  the data.
• High level query and data manipulation language.
• Efficient query processing
• Interface with programming languages

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Describing Data 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.
• 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.
• 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
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Example University Database

• Conceptual schema
• Students(sid string, name string, login
  string,
• age integer, gpareal)
• Courses(cid string, cnamestring,
  creditsinteger)
• Enrolled(sidstring, cidstring, gradestring)
• Physical schema
• Relations stored as unordered files.
• Index on first column of Students.
• External Schema (View)
• Course_info(cidstring,enrollmentinteger)

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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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Objectives of Three-Level Architecture

• All users should be able to access same data.
• A user's view is immune to changes made in other
  views.
• Users should not need to know physical database
  storage details.

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Objectives of Three-Level Architecture

• DBA should be able to change database storage
  structures without affecting the users' views.
• Internal structure of database should be
  unaffected by changes to physical aspects of
  storage.
• DBA should be able to change conceptual structure
  of database without affecting all users.

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ANSI-SPARC Three-level Architecture
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ANSI-SPARC Three-level Architecture

• External Level
• Users' view of the database. Describes that part
  of database that is relevant to a particular
  user.
• Conceptual Level
• Community view of the database. Describes what
  data is stored in database and relationships
  among the data.

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ANSI-SPARC Three-level Architecture

• Internal Level
• Physical representation of the database on the
  computer. Describes how the data is stored in
  the database.

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Differences between Three Levels of ANSI-SPARC
Architecture
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Data Independence

• Logical Data Independence
• Refers to immunity of external schemas to changes
  in conceptual schema.
• Conceptual schema changes e.g. addition/removal
  of entities.
• Should not require changes to external schema or
  rewrites of application programs.

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Data Independence

• Physical Data Independence
• Refers to immunity of conceptual schema to
  changes in the internal schema.
• Internal schema changes e.g. using different file
  organizations, storage structures/devices.
• Should not require change to conceptual or
  external schemas.

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Concurrency Control

• Concurrent execution of user programs key to
  good DBMS performance.
• Disk accesses frequent, pretty slow
• Keep the CPU working on several programs
  concurrently.
• Interleaving actions of different programs
  trouble!
• e.g., deposit withdrawal on same account at
  once
• DBMS ensures such problems dont arise users
  can pretend they are using a single-user system.
• Thank goodness!

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Transaction An Execution of a DB Program

• Key concept is a transaction an atomic sequence
  of database actions (reads/writes).
• Each transaction, executed completely, must take
  the DB from one consistent state to another.
• Users can specify simple integrity constraints on
  the data. The DBMS will enforce these
  constraints.
• Beyond this, the DBMS does not understand the
  semantics of the data. (E.g., it does not
  understand how the interest on a bank account is
  computed).
• Ensuring that a single transaction (run alone)
  preserves consistency is ultimately the users
  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 wasnt 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 its
  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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Structure of a DBMS
These layers must consider concurrency control
and recovery

• 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.

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Query update
User/ Application
Query optimizer
Query execution plan
Execution engine
Record, index requests
Index/record mgr.
Page commands
Buffer manager
Read/write pages
Storage manager
storage
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Advantages of a DBMS

• Data independence
• Efficient data access
• Data integrity security
• Data administration
• Concurrent access, crash recovery
• Reduced application development time
• So why not use them always?
• Can be expensive, complicated to set up and
  maintain
• This cost complexity must be offset by need

- Often worth it!
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Databases make these folks happy ...

• DBMS vendors, programmers
• End users in many fields
• DB application programmers
• Build data entry data analysis tools on top of
  DBMSs
• Database administrators (DBAs)
• Design logical /physical schemas
• Handle 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.
• and...

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Summary, cont.

• DBAs, DB developers hold
• critical jobs and are
• well-paid

• DBMS RD is one of the broadest,
• most exciting areas in CS