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CS457 Object-Oriented Databases Chapters 21-22 as reference. Limitations to the relational model? Examples of applications that will not work well with the relational model?. Shortcomings of DB models for:. CAD/CAM - keep track of 1000\'s of matching parts and subassemblies

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slide1
CS457 Object-Oriented Databases

Chapters 21-22 as reference

slide2
Limitations to the relational model?
  • Examples of applications that will not work well with the relational model?
shortcomings of db models for
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
different because of
Different because of:
  • more complex structures for objects
  • new data types for storing images, text, user defined types
  • nonstandard application specific operations
  • stored procedures (available in some RDBS, such as ORACLE) 
oodb systems
OODB systems
  • prototypes: ORION, IRIS, ENCORE,
  • Commercial products: Postgres, ObjectStore, Versant, Objectivity/DB, O2, Itasca (commercial version of ORION), Gemstone/Opal, POET
slide6
OODB 
  • Some people felt the word object-oriented is too close to OOPL
  • ODB is more generic
different approaches to designing odb
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 
  • Selling point - unified programming and DB language but need query optimization, etc. 
    • Gemstone (Smalltalk), Poet (C++)
designing cont d
Designing cont’d

2. Extend relational DB to include OO features:

  • OID, type hierarchy, inheritance, encapsulation, arbitrary data types, nested objects, etc.
  • Already familiar with DBMS but performance decreased  
  • Postgres - extended Ingres, Oracle
designing cont d1
Designing cont’d

3. Start entire process from scratch (next generation?)

  • unify relational and OO system  
object data model
Object Data Model
  • Bring concepts of OOPL into DB systems
    • Object corresponds to real-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
slide11
ODM

Class has:

  • instances
  • methods and data - encapsulation for information hiding - access only through methods
  • composite classes - composed of multiple types
  • 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
slide12
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
slide13
ODM

Add features such as:

  • concurrency
  • recovery
  • schema evolution
  • Versions
  • What about query language?
  • Performance?
slide14
ODM

Object identity OID

  • correspondence between real-world and database objects
  • used to refer to objects in application programs and in references between objects (relationships) 
  • unique identity to each independent object
oid vs primary key
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
swizzling
Swizzling
  • All references in cached objects replaced with object\'s address
  • pointer to other memory resident objects - swizzling
  • saves OID lookup with subsequent references
slide17
ODM

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
osql object sql
  OSQL – Object SQL 
  • ODMG has provided standards for ODB just like relational databases.
  • Result is OSQL
implementation issues
Implementation Issues:
  • composite objects
  • nested - no joins needed
    • path queries instead of joins
  • pointer (OID) to nested object
  • 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
placing objects on disk
Placing objects on disk?
  • clustering of objects
    • all of same class together
    • subclass follows superclass
  • nested objects
  • indexes for objects? (just like relations)
additional topics
Additional topics
  • schema evolution?
  • 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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