Object Database Management Systems

1
Chapter 18

• Object Database Management Systems

2
Outline

• Motivation for object database management
• Object-oriented principles
• Architectures for object database management
• Object database definition and manipulation in
  SQL1999
• Object database definition and manipulation in
  Oracle 9i

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Motivation Complex Data

• Most relational DBMSs support only a few data
  types.
• Many business applications require large amounts
  of complex data such as images, audio, and video.
  
• The need to integrate complex data with simple
  data drives the demand for object database
  technology.

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Motivation Type System Mismatch

• Increasing use of database access in procedural
  code
• Different data types used in programming
  languages versus DBMSs
• Data type mismatch makes software more difficult
  to develop.
• A relational DBMS cannot perform elementary
  operations on complex data.

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Application Examples

• Dental Office Support
• Real Estate Listing Service
• Auto Insurance Claims

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Object-Oriented Principles

• An object is a combination of data and
  procedures.
• A class is a prototype that defines the variables
  and methods common to all objects of the class.
• Three underlying principles encapsulation,
  inheritance and polymorphism.

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Encapsulation

• Objects can be accessed only through their
  interfaces.
• Classes can be reused rather than just individual
  procedures.
• More complex classes can be defined using simpler
  classes.
• Provides a form of data independence.

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Bond Class Example
CLASS Bond // VARIABLES ATTRIBUTE Float
IntRate ATTRIBUTE Date Maturity // METHODS
Float Yield() // Computes the Bonds Yield
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Inheritance

• Sharing of data and code among similar classes
  (classes and subclasses).
• Inherit variables and methods from parent classes
• When using the subclasses, the methods in the
  parent classes can be used.
• Inheritance provides an improved organization of
  software and incremental reusability.

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Inheritance Examples
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Multiple Inheritance Example
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Polymorphism

• Ability of a computing system to choose among
  multiple implementations
• Benefits
• Fewer, more reusable methods
• incremental modification of code
• Requesting a method execution involves sending a
  message to an object
• Client-server processing and object-oriented
  computing are closely related.

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Processing a Message
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Binding

• Associating an implementation with a message
• Static binding
• Performed at compile-time
• More efficient but less flexible
• Dynamic binding
• Performed at run-time (late binding)
• More flexible but less efficient

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Strong Type Checking

• Complex expressions can involve many methods and
  objects
• Incompatibility errors common in code
• Ability to ensure that programming code contains
  no incompatibility errors
• An important kind of error checking for
  object-oriented coding

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Programming Languages versus DBMSs

• Programming languages have used object-oriented
  principles for many years.
• Programming languages emphasize software
  maintenance and code reusability.
• Object DBMSs are more recent.
• Encapsulation usually is relaxed so that an
  objects data can be referenced in a query.
• Inheritance mechanisms usually are simpler in
  DBMSs

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Architectures for Object Database Management

• Adding object-oriented features to a DBMS is a
  good idea
• Many approaches about the features to add and how
  features should be added.
• Some approaches provide small extensions that
  leave object features outside the DBMS.
• Other approaches involve a complete rewrite of
  the DBMS to accommodate objects
• Marketplace will determine best approaches

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Large Objects and External Software

• Storage of large objects in a database along with
  external software to manipulate large objects.
• Complex data are stored in a field using the BLOB
  (binary large object) data type.
• The large object approach is simple to implement
  and universal.
• The large object approach suffers from serious
  performance drawbacks.

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Large Object Architecture
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Specialized Media Servers

• The use of a dedicated server to manage complex
  data outside of a database.
• Programmers use an application programming
  interface (API) to access complex data.
• Provide good performance for specific kinds of
  complex data.
• The range of operations may be limited.
• May perform poorly when combining simple and
  complex data.

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Specialized Media Server Architecture
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Object database middleware

• The use of middleware to manage complex data
  stored outside of a database along with
  traditional data stored in a database.
• Provides a way to integrate complex data stored
  on PCs and remote servers with relational
  databases.
• Object middleware can suffer performance problems
  because of a lack of integration with a DBMS.

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Object Middleware Approach
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Object Relational DBMS for User-Defined Types

• A relational DBMS extended with an object query
  processor for user-defined data types.
• Complex data is added as a user-defined type.
• User-defined functions can be defined and then
  used in SQL statements.
• SQL1999 provides the standard for object
  relational DBMSs.
• Provide good integration of complex data but
  reliability may be a concern.

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Component Architecture for Object Relational
DBMSs
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Object-Oriented DBMS

• A new kind of DBMS designed especially for
  objects.
• Object-oriented DBMSs have an object query
  processor and an object kernel.
• The Object Data Management Group (ODMG) provides
  the standard for object-oriented DBMSs.

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Component Architecture for Object-Oriented DBMSs
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Summary of Architectures
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Object Database Features in SQL1999

• Very large standard
• Core language part
• Packages
• Details about basic and enhanced object support
• Two levels of conformance

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SQL1999 Packages
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User-Defined Types

• Bundles data and procedures
• Support definition of structured types, not just
  extensions of standard types
• User-defined types can be used as data types for
  columns in tables, passed as parameters, and
  returned as values.
• User-defined functions can be used in expressions
  in the SELECT, the WHERE, and the HAVING clauses.

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User-Defined Type Example
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Explicit Methods

• Return single values and use input parameters
• Implicit first parameter part of user-defined
  type
• CREATE METHOD statement for method body
• Mutation methods change values
• Procedures and functions not associated with types

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Implicit Methods

• Automatically exist for all user-defined types
• Constructor method creates an empty instance
• Observer methods retrieve values
• Mutation methods change values

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User-Defined Type using an Array
Example 2 Polygon type using an ARRAY CREATE
TYPE Polygon AS ( Corners Point ARRAY10,
Color INTEGER ) METHOD Area() RETURNS FLOAT,
-- Computes the area METHOD Scale (Factor FLOAT)
RETURNS Polygon -- Computes a new polygon
scaled by factor NOT FINAL
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Table Definitions

• Traditional style foreign keys to link tables
• Typed tables supports object identifiers and
  object references
• Row type constructor supports rows as variables
  and parameters

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Example of table definition with a row type
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Table Definition with a User-Defined Type
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Subtable Families

• A table can be declared as a subtable of another
  table.
• A subtable inherits the columns of its parent
  tables.
• SQL1999 limits inheritance for tables to single
  inheritance.
• Set inclusion determines the relationship of a
  table to its subtables.

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Subtable Example
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Side effects when manipulating rows in subtable
families

• On insert into a subtable, a corresponding row is
  inserted into each parent table.
• On update in a parent table, the column is also
  updated in all direct and indirect subtables that
  inherit the column.
• On update of an inherited column, the column is
  changed in the corresponding rows of direct and
  indirect parent tables.
• On delete, every corresponding row in both parent
  and subtables is also deleted.

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Manipulating Complex Objects and Subtable
Families

• Path expressions to manipulate columns with row
  references.
• References to methods in expressions using the
  dot notation
• Testing membership in a specific table without
  being a member of any subtables.

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Using the ROW Keyword
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Obtaining Object Identifiers
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Example of path expression versus traditional
expression
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Oracle 9i Object Features

• Supports most parts of the SQL1999 object
  packages
• User-defined types
• Typed tables
• Other object features

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User-Defined Type Example
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Inheritance
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Typed Tables
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Inheritance for Typed Tables
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Inserting into Typed Tables
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Path Expressions
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Other Object Features

• Type substitutability for subtables
• Hierarchical views
• Nested tables

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Summary

• Three principles of object-oriented computing
  guide the development of object DBMSs.
• A number of object DBMS architectures are
  commercially available.
• SQL1999 supports definition and manipulation of
  object relational databases.
• Oracle 9i is a significant implementation of the
  SQL1999 object packages