Object Databases

1
Object Databases

• David Nelson
• CAT
• March 2003

2
Contents

• Relational DBMS
• Suitability, Complex Applications, Inadequacies
• Data Model Generations
• Types of Object Database
• Object Oriented Databases
• OQL
• Object Relational Databases
• SQL3
• Oracle
• Advantages/Disadvantages
• Why more than one model?
• Summary and Further Reading

3
Relational DBMS Suitability

• Relational DBMS are suitable for certain types of
  applications
• simple data types, e.g. dates, strings
• large number of instances, e.g. students,
  employees
• well defined relationships between data, e.g.
  student, course relationships and use of joins
• short transactions, e.g. simple queries
• Most successful for business applications
• On-line transaction processing

4
Complex Applications

• CAD, CAM
• complex objects
• graphics
• a large number of types but few instances of each
  type
• hierarchical design not static
• CASE
• software development lifecycle
• co-operative engineering
• concurrent sharing of design
• code/documentation

5
Complex Applications

• Office Information and Multimedia Systems
• e-mail support
• documentation
• SGML documents
• Geographic Information Systems
• spatial and temporal information, e.g.
  satellite/survey photos, maps
• pattern recognition

6
RDBMS Inadequacies

• Poor separation of real world entities
• normalisation leads to entities that dont
  closely match real world
• joins costly
• Semantic overloading
• all data held as relationships
• no mechanism for differentiation between entities
  and relationships
• Poor support for integrity and enterprise
  constraints
• relational systems good for supporting
  referential, entity and simple business
  constraints
• not good for more complex enterprise constraints

7
RDBMS Inadequacies

• Homogeneous data structure
• data pushed into rows and columns
• not all real world data can be organised in this
  way
• Limited operations
• SQL does not allow new operations to be defined
• e.g. select age from person
• Difficulty handling recursive queries
• e.g. find all ancestors

8
RDBMS Inadequacies

• Impedance mismatch
• need to embed SQL to get computational
  completeness
• data types in SQL and programming language dont
  match
• Concurrency, schema changes and poor navigational
  access
• no support for long duration transactions
• difficult to change schema, e.g. add columns to a
  table
• RDBMS based on content based access
• Not navigational

9
Data Models

• 1st Generation
• 2nd generation
• 3rd
• generation

10
Object-Oriented Databases

• Object-Oriented Database
• e.g. ObjectStore, Objectivity, Jasmine, POET
• based on object-oriented programming techniques
• include concepts such as
• user extensible type system, complex objects,
  encapsulation, inheritance, polymorphism, dynamic
  binding, object identity
• ODMG standard being devised to define data model
  and query language standard
• also defines interoperability between ODMG
  compliant systems

11
OQL Example ODL Schema

• class Student
• (extent students)
• attribute short id
• attribute string name
• attribute string address
• attribute date dob
• relationship setltModulegt takes
• inverse Module takenby
• short age()

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OQL Example ODL Schema

• class Module
• (extent modules)
• attribute string title
• attribute short semester
• relationship setltStudentgt takenby
• inverse Student takes
• class Postgrad extends Student
• (extent postgrads)
• attribute string thesis_title

13
Object Query Language

• e.g. select distinct x.age
• from Persons x
• where x.name Pat
• Return literal of type setltstructgt
• select distinct struct(a x.age, s x.sex)
• from Persons x
• where x.name Pat

14
OQL Examples

• Path Expressions
• select c.address
• from Persons p, p.children c
• where p.address.street Main Street
• and count(p.children) gt 2
• and c.address.city ! p.address.city
• Methods
• select max(select c.age from p.children c)
• from Persons p
• where p.name Paul

15
Object-Relational Databases

• Object-Relational Database
• e.g. Oracle 8, Informix, Illustra
• extend relational model with OO features
• e.g. polymorphism, complex objects, extensibility
• but keep relationally based
• SQL99 standard developed as an object-relational
  version of SQL92
• Previously known as SQL-3

16
SQL99

• Row types
• a data type that can represent types of rows in
  tables
• e.g.
• CREATE TABLE branch(
• Bno VARCHAR(3),
• address ROW(
• street VARCHAR(25),
• town VARCHAR(15),
• pcode ROW( city_id VARCHAR(4)
• subpart VARCHAR(4))))
• INSERT INTO branch
• VALUES(B5, (22 Deer Rd, Sidcup, (SW1,
  4EH)))

17
SQL99

• UDTs
• abstract data types
• consists of one or more attribute defns
• encapsulation supported
• CREATE TYPE person_type AS (
• PRIVATE
• date_of_birth DATE CHECK(date_of_birth gt DATE
  1990-01-01)
• PUBLIC
• fname VARCHAR(15) NOT NULL,
• lname VARCHAR(15) NOT NULL,
• FUNCTION get_age (P person_type) RETURNS
  INTEGER
• RETURN / code to calc age /
• END ...
• END) NOT FINAL

18
SQL99

• User defined routines (UDR)
• may be defined as part of a UDT or as part of a
  schema
• can be a procedure, function or iterative routine
• Can be written in SQL or in an external
  programming language

19
SQL99 Examples

• Querying
• uses SQL92 syntax with extensions to handle
  objects
• e.g.
• SELECT s.lname, s.get_age
• FROM staff s
• WHERE s.is_manager
• SELECT p.lname, p.address
• FROM person p
• WHERE p.get_age gt 65
• SELECT p.lname, p.address
• FROM ONLY (person) p
• WHERE p.get_age gt 65

20
SQL99

• Reference Types and OID
• system generated, type REF
• reference types can be used to define
  relationships between row types
• reference types uniquely identify rows
• allows rows to be shared across tables
• complex joins can be replaced by simple path
  expressions
• reference types do not provide referential
  integrity
• Collection types
• ARRAYs, LISTs, SETs, MULTISETs

21
SQL99

• Persistent Stored Modules
• SQL3 now computationally complete
• New statements added
• Assignment
• IF .. THEN .. ELSE .. ENDIF, and CASE
• REPEAT BLOCKS
• CALL and RETURN for invoking procedures
• Condition handling
• Triggers
• Triggering events include insertion, deletion and
  update of rows in a table

22
Oracle 8

• An object-relational extension to Oracle 7
• Object types can be used to create object tables
  with object identifiers
• attributes
• methods
• Does not support object or table hierarchies
• Oracle 9 supports object hierarchies
• Postgres supports table hierarchies
• New types
• VARRAYs and nested tables
• REFs
• LOBs

23
Object Database Advantages

• Enriched modelling capabilities
• can model both state and behaviour
• models real world more naturally
• Extensibility
• ability to build new types and use in system
• abstract data types
• Removal of impedance mismatch
• single language interface between DML and
  programming language
• computationally complete

24
Object Database Advantages

• More expressive query language
• support for navigational queries
• support for methods
• Support for schema evolution
• tight coupling between data and applications
• generalisation, inheritance
• Support for long duration transactions
• RDBMS enforce serializability

25
Object Database Advantages

• Applicability to advanced database applications
• CAD, CAM, GIS, etc.
• Improved performance
• improved performance for engineering problems
• but different type of problem

26
Object Database Disadvantages

• Lack of universal data model
• RDBs based on set theory
• OODBs lack theoretical basis
• Lack of experience
• limited use
• steep learning curve
• geared towards programmers rather than typical
  users
• Lack of standards
• ODMG standard evolving for standard data model
  and standard query language

27
Object Database Disadvantages

• Query optimisation compromises encapsulation
• need to break encapsulation to optimise queries
• e.g. access a private attribute to speed-up query
• Locking at object level may impact performance
• e.g. locking large inheritance hierarchies
• Complexity
• expensive
• difficult to use

28
Object Database Disadvantages

• Lack of support for views
• an important concept in RDBs
• provide customisable views of a database
• Lack of support for security
• no views
• coarse granularity
• difficult to grant access rights on individual
  objects/classes

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Object-Relational Advantages

• Weaknesses of RDBMS
• Reuse and Sharing
• extending the DBMS server to perform standard
  functionality centrally
• functionality shared by all applications
• Evolutionary rather than revolutionary
• SQL3 upwardly compatible with current SQL
  standard

30
Object-Relational Disadvantages

• Complexity and Associated Increased Costs
• simplicity and purity of relational model is lost
• majority of applications do not achieve optimal
  performance
• Semantic gap between object-oriented and
  relational
• OO applications not as data centric as Relational
• Objectives of Initial SQL standard were to
  minimise user effort and be easy to learn

31
Why more than one model?

• Stonebraker proposed a four quadrant view of the
  database world

Search capabilities/ multi-user support
Data complexity/extensibility
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Summary

• More than one data model, but each model good for
  particular types of application
• Each has its own advantages and disadvantages
• Object-Relational DBMS are taking over Relational
  DBMS in market place
• e.g. Oracle 8
• Object-Oriented still have their own niche
• E.g. CAD, CASE, etc.

33
Further Reading

• Connelly and Begg, chapters 21-23
• a very good introduction to object-oriented and
  object-relational databases
• Cattell, et. al. The Object Data Standard, 3rd
  ed.
• Discusses the ODMG standard for object databases
• Stonebraker, Object-Relational Databases The
  Next Great Wave
• written by the person who produced the first
  object-relational database Postgres

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Further Reading

• OO Manifesto
• http//citeseer.nj.nec.com/atkinson89objectoriente
  d.html
• OR Manifesto
• http//citeseer.nj.nec.com/for90thirdgeneration.ht
  ml