ObjectOriented Databases

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• Object-Oriented Databases
• Chapters 21-22 as reference

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• Limitations to the relational model?
• Examples of applications that will not work well
  with the relational model?

3
Shortcomings of DB models for

• CAD/CAM - keep track of 1000's of matching parts
  and subassemblies
• relational inefficient for this
• object versioning
• complex market-oriented operations (securities)
• geographical information systems
• CASE
• OA (office automation)
• multimedia databases
• WWW

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Different because of

• requirements and characteristics that differ from
  traditional business applications
• more complex structures for objects
• new data types for storing images, text, user
  defined types
• nonstandard application specific operations
• stored procedures (also available now in some
  RDBS, such as ORACLE)

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OODB systems

• prototypes ORION, IRIS, ENCORE,
• Commercial products ObjectStore, Versant,
  Objectivity/DB, Itasca (commercial version of
  ORION), POET
• Open source object database
• Postgres
• Perst (McObject) supports SQL, ACID, etc.
• Db4o queries only in Java, C, VB.NET
• Magma Squeak (smalltalk-Gemstone)

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OODB 

• Some people felt the word object-oriented is too
  close to OOPL
• ODB is more generic

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Different approaches to designing ODB

• 1. Applications written in extension of existing
  OOPL (1st generation OODB)
• language, compiler, execution environment, etc.
  extended to incorporate 
• DB functionality
• store and manage objects created by OOPL
• DB features such as transactions, concurrency
  control, etc.  
• Selling point - unified programming and DB
  language but need query optimization, etc. 
• Gemstone (Smalltalk), Poet (C)

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Designing contd

• 2. Extend relational DB to include OO
  features
• OID, type hierarchy, inheritance, encapsulation,
  arbitrary data types, nested objects, etc.
• Query language extended with programming
  capability
• application communicates with DBMS through
  embedded query language calls in application
  program
• Already familiar with DBMS but performance
  decreased  
• Postgres - extended Ingres, Oracle

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Designing contd

• 3. Start entire process from scratch (next
  generation?)
• unify relational and OO system  

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Object Data Model

• Bring concepts of OOPL into DB systems
• Object corresponds to mini-world object
• Object is data and behavior, object has
  attributes and operations
• Data object has OID - immutable
• Group data objects into classes - abstract
  mechanism, share same structure and behavior

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ODM

• Class has
• instances
• methods and data - encapsulation for information
  hiding - access only through methods
• composite classes - composed of multiple types
• Types vs. classes
• nested objects - contains another object
• complex objects - set of objects      
• class hierarachy (ISA) specialization - define
  new classes from existing ones
• inheritance of attributes and methods - reuse

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ODM

• Completeness
• DBS needs to be computationally complete (Turing)
• SQL not computationally complete - unless
  embedded SQL - impedance mismatch, since sets
• connections with DML and OOPL in ODB more
  acceptable

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ODM

• Add features such as
• concurrency
• recovery
• schema evolution
• Versions
• What about query language?
• Performance?

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ODM

• Object identity OID
• correspondence between mini-world and database
  objects
• used to refer  to objects in application programs
  and in references between objects (relationships)
  
• unique over entire DB and even over distributed
  DB

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OID vs. primary key

• identity based vs. value-based
• unique over entire DB and even over distributed
  DB (if primary key changes, still same real-world
  object)
• immutable - values of OID for object should not
  change
• - OID not assigned to another object
• - not dependent on any attribute
  values
• - not use physical address      
• system generated OID
• not visible to user

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OIDs in reality

• in extended ODMSs, query language extended to
  allow access by OID
• in OOPL, allow value based access by key
• in Postgres - oid attribute in every tuple

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Swizzling

• All references in cached objects replaced with
  object's address
• pointer to other memory resident objects -
  swizzling
• saves OID lookup with subsequent references

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Encapsulation

• Encapsulation
• information and operations, structure and
  behavior
• implementation hidden
• define behavior of object using operations
• object accessible through predefined operations -
  methods
• method invoked by sending messages
• If query on attributes? SQL violates
  encapsulation

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  Query Languge?OSQL Object SQL 

• ODMG has provided standards for ODB just like
  relational databases.
• Result is OSQL (object SQL)

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Implementation Issues

• pointer (OID) to nested object
• nested - no joins needed
• path queries instead of joins
• when bring in an object into memory, bring in
  nested objects as well
• replace OID with memory address (can't do this in
  relational)
• eager and lazy swizzling
• fast access - memory resident
•  

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Placing objects on disk?

• clustering of objects
• all of same class together
• subclass follows superclass
• nested objects
• indexes for objects? (just like relations)
• Views?

22
Performance

• Performance of OODB affected by         cache
  sizes         clustering         indexes
          swizzling         replication

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OODB issues

• physical OODB design
• clustering problems (NP-hard)
• placing objects onto pages so number of page
  faults is minimal
• user hints for clustering
• where to stores objects, near related objects
• how to store objects, near related objects

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Example

• Bob is a person, but also an employee
•   store all persons then employee
•   Bob Sally Joe Bill           Bob
• if store employee attributes, must also retrieve
  information from person class

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Example

• Or can store all persons together, then students,
  then employees
• person    students          employee
• Bill        Sally Joe Bob      
• Must retrieve all of them for person, and remove
  any extra attributes

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Additional Topics

• schema evolution?
• Definition of class, changes to structure
• views?
• Extended relational
• Now object-relational
• Examine Oracle documentation to see how OO
  features included in 9i
• Can create both tables and objects

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Object-relational - ORDMS

• Informix, IBM, Hewlett-Packard, Unisys, Oracle,
  and UniSQL
• Extended relational" is probably the more
  appropriate term
• ORDMSs specified by the extensions to the SQL
  standard, SQL3/4

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ORDMS

• Superset of RDMS
• Definition?
• Allows user defined types
• User defined functions
• Indexing/access methods to support them
• ORDBMSs have had their greatest success in
  managing media objects and complex data such as
  geospatial and financial time series data
• frequently used in Web applications and
  specialized data warehouses

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Oracles object relational

• Oracle ORDB
• Relaxes 1NF
• Has object types
• Allows users to create
• Types as objects
• Object tables

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Create type

• Create type must be followed by a / in SQL Plus
  ( optional)
• No objects created, like a struct type
• // Create object type
• Create type name_t as object
• ( lname varchar(10),
• fname varchar(10),
• mi char(1) )
• // Create relational table
• Create table teachers
• ( tid int,
• tname name_t,
• room int)

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How to insert?

• Can use object_type_name( ) form as an object
  constructor,
• Builds objects from value of attributes
• Insert into teachers values (1234, name_t(Sky,
  Sue, V), 120)

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Path queries

• Select t.tname.lname from teachers t where room
  120
• // sometimes aliases are required
• // best to always use

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Create Object table

• Create type person_t as object
• (ssn int,
• Pname name_t,
• age int)
• Create table people of person_t
• //Can use value ( )
• // The following are similar
• Select value(p) from people p where age gt 25
• Select from people where age gt 25

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Inheritance

• CREATE TYPE vehicle AS OBJECT
• (VID VARCHAR2(20),
• COLOR CHAR(10),
• MAKE VARCHAR2(10),
• MODEL VARCHAR2(10) ) NOT FINAL
• CREATE TYPE car UNDER vehicle
• ( DOORS NUMBER,
• SPORT CHAR(1) )

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Ref Object reference

• Use REF keyword to point to a row object
• Use dot notation to follow a REF to derefence a
  REF
• If you want to query the OID use
• Select REF(t) from toys t
• How to insert tuples and query
• Example of object-relational in Oracle
•