Chapter 18 Object Database Management Systems

Outline Motivation for object database management Object-oriented principles Architectures for object database management Object database definition and manipulation in SQL:1999 Object database definition and manipulation in Oracle 9i

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.

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.

Application Examples Dental Office Support Real Estate Listing Service Auto Insurance Claims

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.

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.

Bond Class Example CLASS Bond { // VARIABLES: ATTRIBUTE Float IntRate; ATTRIBUTE Date Maturity; // METHODS: Float Yield(); // Computes the Bond’s Yield };

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.

Inheritance Examples

Multiple Inheritance Example

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.

Processing a Message

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

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

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 object’s data can be referenced in a query. Inheritance mechanisms usually are simpler in DBMSs

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

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.

Large Object Architecture

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.

Specialized Media Server Architecture

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.

Object Middleware Approach

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. SQL:1999 provides the standard for object relational DBMSs. Provide good integration of complex data but reliability may be a concern.

Component Architecture for Object Relational DBMSs

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.

Component Architecture for Object-Oriented DBMSs

Summary of Architectures

Object Database Features in SQL:1999 Very large standard Core language part Packages Details about basic and enhanced object support Two levels of conformance

SQL:1999 Packages

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.

User-Defined Type Example

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

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

User-Defined Type using an Array Example 2: Polygon type using an ARRAY CREATE TYPE Polygon AS ( Corners Point ARRAY[10], Color INTEGER ) METHOD Area() RETURNS FLOAT, -- Computes the area METHOD Scale (Factor FLOAT) RETURNS Polygon -- Computes a new polygon scaled by factor NOT FINAL;

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

Example of table definition with a row type

Table Definition with a User- Defined Type

Subtable Families A table can be declared as a subtable of another table. A subtable inherits the columns of its parent tables. SQL:1999 limits inheritance for tables to single inheritance. Set inclusion determines the relationship of a table to its subtables.

Subtable Example

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.

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.

Using the ROW Keyword

Obtaining Object Identifiers

Example of path expression versus traditional expression

Oracle 9i Object Features Supports most parts of the SQL:1999 object packages User-defined types Typed tables Other object features

User-Defined Type Example

Inheritance

Typed Tables

Inheritance for Typed Tables

Inserting into Typed Tables

Path Expressions

Other Object Features Type substitutability for subtables Hierarchical views Nested tables

Summary Three principles of object-oriented computing guide the development of object DBMSs. A number of object DBMS architectures are commercially available. SQL:1999 supports definition and manipulation of object relational databases. Oracle 9i is a significant implementation of the SQL:1999 object packages