Oodbms ch 20

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Oodbms ch 20

  1. 1. Chapter 20 Concepts forObject-Oriented Databases Copyright © 2004 Pearson Education, Inc.
  2. 2. Chapter Outline20.1 Overview of O-O Concepts20.2 O-O Identity, Object Structure and Type Constructors20.3 Encapsulation of Operations, Methods and Persistence20.4 Type and Class Hierarchies and Inheritance20.5 Complex Objects20.6 Other O-O Concepts20.7 Summary & Current Status Elmasri/Navathe, Fundamentals of Database Systems, Fourth Edition Chapter 20-2 Copyright © 2004 Ramez Elmasri and Shamkant Navathe
  3. 3. Object Oriented Concepts Object – An object represents a real world entity (Person), a concept (Cricket Match), a logical thing (Driving Licence), a physical thing (Car) – User Defined Complex Data type – An object is made of two things: State: properties (name, address, birthDate of a Person) Behaviour: operations (age of a Person is computed from birthDate and current date) Elmasri/Navathe, Fundamentals of Database Systems, Fourth Edition Copyright © 2004 Ramez Elmasri and Shamkant Navathe
  4. 4. Object Oriented Concepts An object has associated with it: – A set of variables that contain the data for the object. The value of each variable is itself an object. – A set of messages to which the object responds; each message may have zero, one, or more parameters. – A set of methods, each of which is a body of code to implement a message; a method returns a value as the response to the message The physical representation of data is visible only to the implementor of the object Messages and responses provide the only external interface to an object. The term message does not necessarily imply physical message passing. Messages can be implemented as procedure invocations. Elmasri/Navathe, Fundamentals of Database Systems, Fourth Edition Copyright © 2004 Ramez Elmasri and Shamkant Navathe
  5. 5. Object Oriented Concepts Methods are programs written in general-purpose language with the following features – only variables in the object itself may be referenced directly – data in other objects are referenced only by sending messages. Methods can be read-only or update methods – Read-only methods do not change the value of the object Elmasri/Navathe, Fundamentals of Database Systems, Fourth Edition Copyright © 2004 Ramez Elmasri and Shamkant Navathe
  6. 6. Object Oriented Concepts Strictly speaking, every attribute of an entity must be represented by a variable and two methods, called the accessor methods, one to read and the other to update the attribute – e.g., the attribute address is represented by a variable address and two messages get-address and set-address. – For convenience, many object-oriented data models permit direct access to variables of other objects. Elmasri/Navathe, Fundamentals of Database Systems, Fourth Edition Copyright © 2004 Ramez Elmasri and Shamkant Navathe
  7. 7. Object Oriented Concepts Object – Every object has a unique Object Identifier (OID) – Described by 4 major characteristics Identifier : System wide unique id of an object Name : optionally, may have a unique name in a DB Lifetime: determines if the object is persistent or transient Structure: determines the construction using type constructors Elmasri/Navathe, Fundamentals of Database Systems, Fourth Edition Copyright © 2004 Ramez Elmasri and Shamkant Navathe
  8. 8. Object Identifiers Object identifiers used to uniquely identify objects ( 32 or 64 bit integers) – Object identifiers are unique:  no two objects have the same identifier  each object has only one object identifier – can be stored as a field of an object, to refer to another object. – Can be  system generated (created by database) or  external (such as social-security number) – System generated identifiers:  Are easier to use, but cannot be used across database systems  May be redundant if unique identifier already exists Elmasri/Navathe, Fundamentals of Database Systems, Fourth Edition Copyright © 2004 Ramez Elmasri and Shamkant Navathe
  9. 9. Object Oriented ConceptsObject Structure The state (current value) of a complex object may be constructed from other objects (or other values) by using certain type constructors Can be represented by (i,c,v) – i is an unique id – c is a type constructor – v is the object state Constructors – Basic types: atom, tuple and set – Collection type: list, bag and array Elmasri/Navathe, Fundamentals of Database Systems, Fourth Edition Copyright © 2004 Ramez Elmasri and Shamkant Navathe
  10. 10. Object Identity, Object Structure, and Type Constructors (3) Example 1, one possible relational database state corresponding to COMPANY schema Elmasri/Navathe, Fundamentals of Database Systems, Fourth Edition Chapter 20-10 Copyright © 2004 Ramez Elmasri and Shamkant Navathe
  11. 11. Object Identity, Object Structure, and Type Constructors (4) Example 1 (cont.): Elmasri/Navathe, Fundamentals of Database Systems, Fourth Edition Chapter 20-11 Copyright © 2004 Ramez Elmasri and Shamkant Navathe
  12. 12. Object Identity, Object Structure, and Type Constructors (5) Example 1 (cont.) Elmasri/Navathe, Fundamentals of Database Systems, Fourth Edition Chapter 20-12 Copyright © 2004 Ramez Elmasri and Shamkant Navathe
  13. 13. Object Identity, Object Structure, and Type Constructors (6) Example 1 (cont.) We use i1, i2, i3, . . . to stand for unique system- generated object identifiers. Consider the following objects: o1 = (i1, atom, ‘Houston’) o2 = (i2, atom, ‘Bellaire’) o3 = (i3, atom, ‘Sugarland’) o4 = (i4, atom, 5) o5 = (i5, atom, ‘Research’) o6 = (i6, atom, ‘1988-05-22’) o7 = (iElmasri/Navathe,i2, i3}) 7 , set, {i1, Fundamentals of Database Systems, Fourth Edition Chapter 20-13 Copyright © 2004 Ramez Elmasri and Shamkant Navathe
  14. 14. Object Identity, Object Structure, and Type Constructors (7) Example 1(cont.) o8 = (i8, tuple, <dname:i5, dnumber:i4, mgr:i9, locations:i7, employees:i10, projects:i11>) o9 = (i9, tuple, <manager:i12, manager_start_date:i6>) o10 = (i10, set, {i12, i13, i14}) o11 = (i11, set {i15, i16, i17}) o12 = (i12, tuple, <fname:i18, minit:i19, lname:i20, ssn:i21, . . ., salary:i26, supervi­sor:i27, dept:i8>) ... Elmasri/Navathe, Fundamentals of Database Systems, Fourth Edition Chapter 20-14 Copyright © 2004 Ramez Elmasri and Shamkant Navathe
  15. 15. Object Identity, Object Structure, and Type Constructors (8)Example 1 (cont.)The first six objects listed in this example represent atomic values. Object seven is a set-valued object that represents the set of locations for department 5; the set refers to the atomic objects with values {‘Houston’, ‘Bellaire’, ‘Sugarland’}. Object 8 is a tuple-valued object that represents department 5 itself, and has the attributes DNAME, DNUMBER, MGR, LOCATIONS, and so on. Elmasri/Navathe, Fundamentals of Database Systems, Fourth Edition Chapter 20-15 Copyright © 2004 Ramez Elmasri and Shamkant Navathe
  16. 16. Object Identity, Object Structure, and Type Constructors (9)Example 2: This example illustrates the difference between the two definitions for comparing object states for equality. o1 = (i1, tuple, <a1:i4, a2:i6>) o2 = (i2, tuple, <a1:i5, a2:i6>) o3 = (i3, tuple, <a1:i4, a2:i6>) o4 = (i4, atom, 10) o5 = (i5, atom, 10) o6 = (i6, atom, 20) Elmasri/Navathe, Fundamentals of Database Systems, Fourth Edition Chapter 20-16 Copyright © 2004 Ramez Elmasri and Shamkant Navathe
  17. 17. Object Identity, Object Structure, and Type Constructors (10)Example 2 (cont.):In this example, The objects o1 and o2 have equal states,since their states at the atomic level are the same but thevalues are reached through distinct objects o4 and o5.However, the states of objects o1 and o3 are identical,even though the objects themselves are not because theyhave distinct OIDs. Similarly, although the states of o4and o5 are identical, the actual objects o4 and o5 are equalbut not identical, because they have distinct OIDs. Elmasri/Navathe, Fundamentals of Database Systems, Fourth Edition Chapter 20-17 Copyright © 2004 Ramez Elmasri and Shamkant Navathe
  18. 18. Object Identity, Object Structure, and Type Constructors (11)Figure 20.1 Representation of a DEPARTMENT complex object as a graph Elmasri/Navathe, Fundamentals of Database Systems, Fourth Edition Chapter 20-18 Copyright © 2004 Ramez Elmasri and Shamkant Navathe
  19. 19. Object Identity, Object Structure, and Type Constructors (12)Figure 20.2 Specifying the object types Employee, date, and Department using type constructors Elmasri/Navathe, Fundamentals of Database Systems, Fourth Edition Chapter 20-19 Copyright © 2004 Ramez Elmasri and Shamkant Navathe
  20. 20. 20.3 Encapsulation of Operations, Methods, and Persistence (1)Encapsulation One of the main characteristics of OO languages and systems Related to the concepts of abstract data types and information hiding in programming languages Elmasri/Navathe, Fundamentals of Database Systems, Fourth Edition Chapter 20-20 Copyright © 2004 Ramez Elmasri and Shamkant Navathe
  21. 21. Encapsulation of Operations, Methods, and Persistence (2) Specifying Object Behavior via Class Operations: The main idea is to define the behavior of a type of object based on the operations that can be externally applied to objects of that type. In general, the implementation of an operation can be specified in a general-purpose programming language that provides flexibility and power in defining the operations. Elmasri/Navathe, Fundamentals of Database Systems, Fourth Edition Chapter 20-21 Copyright © 2004 Ramez Elmasri and Shamkant Navathe
  22. 22. Encapsulation of Operations, Methods, and Persistence (3) Specifying Object Behavior via Class Operations (cont.): For database applications, the requirement that all objects be completely encapsulated is too stringent. One way of relaxing this requirement is to divide the structure of an object into visible and hidden attributes (instance variables). Elmasri/Navathe, Fundamentals of Database Systems, Fourth Edition Chapter 20-22 Copyright © 2004 Ramez Elmasri and Shamkant Navathe
  23. 23. Encapsulation of Operations, Methods, and Persistence (4)Figure 20.3 Adding operations to definitions ofEmployee and Department Elmasri/Navathe, Fundamentals of Database Systems, Fourth Edition Chapter 20-23 Copyright © 2004 Ramez Elmasri and Shamkant Navathe
  24. 24. Encapsulation of Operations, Methods, and Persistence (5)Specifying Object Persistence via Naming and Reachability: Naming Mechanism: Assign an object a unique persistent name through which it can be retrieved by this and other programs. Reachability Mechanism: Make the object reachable from some persistent object. An object B is said to be reachable from an object A if a sequence of references in the object graph lead from object A to object B. Elmasri/Navathe, Fundamentals of Database Systems, Fourth Edition Chapter 20-24 Copyright © 2004 Ramez Elmasri and Shamkant Navathe
  25. 25. Encapsulation of Operations, Methods, and Persistence (6)Specifying Object Persistence via Naming and Reachability (cont.): In traditional database models such as relational model or EER model, all objects are assumed to be persistent. In OO approach, a class declaration specifies only the type and operations for a class of objects. The user must separately define a persistent object of type set (DepartmentSet) or list (DepartmentList) whose value is the collection of references to all persistent DEPARTMENT objects Elmasri/Navathe, Fundamentals of Database Systems, Fourth Edition Chapter 20-25 Copyright © 2004 Ramez Elmasri and Shamkant Navathe
  26. 26. Encapsulation of Operations, Methods, and Persistence (7)Note: The above figure is now called Figure 20.4 in Edition 4 Elmasri/Navathe, Fundamentals of Database Systems, Fourth Edition Chapter 20-26 Copyright © 2004 Ramez Elmasri and Shamkant Navathe
  27. 27. 20.4 Type and Class Hierarchies and Inheritance (1) Type (class) Hierarchy A type in its simplest form can be defined by giving it a type name and then listing the names of its visible (public) functions When specifying a type in this section, we use the following format, which does not specify arguments of functions, to simplify the discussion: TYPE_NAME: function, function, . . . , function Example: PERSON: Name, Address, Birthdate, Age, SSN Elmasri/Navathe, Fundamentals of Database Systems, Fourth Edition Chapter 20-27 Copyright © 2004 Ramez Elmasri and Shamkant Navathe
  28. 28. Type and Class Hierarchies and Inheritance (2) Subtype: when the designer or user must create a new type that is similar but not identical to an already defined type Supertype: It inherits all the functions of the subtype Elmasri/Navathe, Fundamentals of Database Systems, Fourth Edition Chapter 20-28 Copyright © 2004 Ramez Elmasri and Shamkant Navathe
  29. 29. Type and Class Hierarchies and Inheritance (3)Example (1): EMPLOYEE: Name, Address, Birthdate, Age, SSN, Salary, HireDate, Seniority STUDENT: Name, Address, Birthdate, Age, SSN, Major, GPA OR: EMPLOYEE subtype-of PERSON: Salary, HireDate, Seniority STUDENT subtype-of PERSON: Major, GPA Elmasri/Navathe, Fundamentals of Database Systems, Fourth Edition Chapter 20-29 Copyright © 2004 Ramez Elmasri and Shamkant Navathe
  30. 30. Type and Class Hierarchies and Inheritance (4)Example (2): Consider a type that describes objects in plane geometry, which may be defined as follows: GEOMETRY_OBJECT: Shape, Area, ReferencePoint Elmasri/Navathe, Fundamentals of Database Systems, Fourth Edition Chapter 20-30 Copyright © 2004 Ramez Elmasri and Shamkant Navathe
  31. 31. Type and Class Hierarchies and Inheritance (5) Example (2) (cont.): Now suppose that we want to define a number of subtypes for the GEOMETRY_OBJECT type, as follows: RECTANGLE subtype-of GEOMETRY_OBJECT: Width, Height TRIANGLE subtype-of GEOMETRY_OBJECT: Side1, Side2, Angle CIRCLE subtype-of GEOMETRY_OBJECT: Radius Elmasri/Navathe, Fundamentals of Database Systems, Fourth Edition Chapter 20-31 Copyright © 2004 Ramez Elmasri and Shamkant Navathe
  32. 32. Type and Class Hierarchies and Inheritance (6) Example (2) (cont.): An alternative way of declaring these three subtypes is to specify the value of the Shape attribute as a condition that must be satisfied for objects of each subtype: RECTANGLE subtype-of GEOMETRY_OBJECT (Shape=‘rectangle’): Width, Height TRIANGLE subtype-of GEOMETRY_OBJECT (Shape=‘triangle’): Side1, Side2, Angle CIRCLE subtype-of GEOMETRY_OBJECT (Shape=‘circle’): Radius Elmasri/Navathe, Fundamentals of Database Systems, Fourth Edition Chapter 20-32 Copyright © 2004 Ramez Elmasri and Shamkant Navathe
  33. 33. Type and Class Hierarchies and Inheritance (7) Extents: In most OO databases, the collection of objects in an extent has the same type or class. However, since the majority of OO databases support types, we assume that extents are collections of objects of the same type for the remainder of this section. Persistent Collection: It holds a collection of objects that is stored permanently in the database and hence can be accessed and shared by multiple programs Elmasri/Navathe, Fundamentals of Database Systems, Fourth Edition Chapter 20-33 Copyright © 2004 Ramez Elmasri and Shamkant Navathe
  34. 34. Type and Class Hierarchies and Inheritance (8) Transient Collection: It exists temporarily during the execution of a program but is not kept when the program terminatesclass Employee (extent all_emp key ssn) { attribute string name; attribute string ssn; attribute short age; relationship Dept works_for inverse dept :: has; void reassign (in string new_name); }; Elmasri/Navathe, Fundamentals of Database Systems, Fourth Edition Chapter 20-34 Copyright © 2004 Ramez Elmasri and Shamkant Navathe
  35. 35. EER DIAGRAM Person dStudent Lecturer N worksFor 1 Department Elmasri/Navathe, Fundamentals of Database Systems, Fourth Edition Chapter 20-35 Copyright © 2004 Ramez Elmasri and Shamkant Navathe
  36. 36. University schema in ODL class Person { attribute struct pname {string fname, string lname, string mname}name; attribute string address; attribute date birthDate; short age(); }; class Lecturer extends Person (extent Lecturers) { attribute short room; relationship Department worksFor inverse Department::staff; boolean teachUnit(in Unit U); }; class Student extends Person (extent Students) { attribute string major; boolean register(in Course C); boolean takeUnit(in Unit U); };class Department (extent Departments){ attribute string name; relationship set<Lecturer> staff inverse Lecturer::worksFor; }; Elmasri/Navathe, Fundamentals of Database Systems, Fourth Edition Copyright © 2004 Ramez Elmasri and Shamkant Navathe
  37. 37. 20.5 Complex Objects (1) Unstructured complex object: It is provided by a DBMS and permits the storage and retrieval of large objects that are needed by the database application. Typical examples of such objects are bitmap images and long text strings (such as documents); they are also known as binary large objects, or BLOBs for short. This has been the standard way by which Relational DBMSs have dealt with supporting complex objects, leaving the operations on those objects outside the RDBMS. Elmasri/Navathe, Fundamentals of Database Systems, Fourth Edition Chapter 20-37 Copyright © 2004 Ramez Elmasri and Shamkant Navathe
  38. 38. Complex Objects (2) Structured complex object: It differs from an unstructured complex object in that the object’s structure is defined by repeated application of the type constructors provided by the OODBMS. Hence, the object structure is defined and known to the OODBMS. The OODBMS also defines methods or operations on it. Elmasri/Navathe, Fundamentals of Database Systems, Fourth Edition Chapter 20-38 Copyright © 2004 Ramez Elmasri and Shamkant Navathe
  39. 39. 20.6 Other Objected-Oriented Concepts (1) Polymorphism (Operator Overloading): This concept allows the same operator name or symbol to be bound to two or more different implementations of the operator, depending on the type of objects to which the operator is applied Elmasri/Navathe, Fundamentals of Database Systems, Fourth Edition Chapter 20-39 Copyright © 2004 Ramez Elmasri and Shamkant Navathe
  40. 40. Other Objected-Oriented Concepts (2) Multiple Inheritance and Selective Inheritance Multiple inheritance in a type hierarchy occurs when a certain subtype T is a subtype of two (or more) types and hence inherits the functions (attributes and methods) of both supertypes.For example, we may create a subtype ENGINEERING_MANAGER that is a subtype of both MANAGER and ENGINEER. This leads to the creation of a type lattice rather than a type hierarchy. Elmasri/Navathe, Fundamentals of Database Systems, Fourth Edition Chapter 20-40 Copyright © 2004 Ramez Elmasri and Shamkant Navathe
  41. 41. Other Objected-Oriented Concepts (3)Versions and Configurations Many database applications that use OO systems require the existence of several versions of the same object There may be more than two versions of an object. Elmasri/Navathe, Fundamentals of Database Systems, Fourth Edition Chapter 20-41 Copyright © 2004 Ramez Elmasri and Shamkant Navathe
  42. 42. Other Objected-Oriented Concepts (4) Configuration: A configuration of the complex object is a collection consisting of one version of each module arranged in such a way that the module versions in the configuration are compatible and together form a valid version of the complex object. Elmasri/Navathe, Fundamentals of Database Systems, Fourth Edition Chapter 20-42 Copyright © 2004 Ramez Elmasri and Shamkant Navathe
  43. 43. Elmasri/Navathe, Fundamentals of Database Systems, Fourth Edition Copyright © 2004 Ramez Elmasri and Shamkant Navathe
  44. 44. RDBMS – a view point RDBMS representation of a pet called “cat’ Elmasri/Navathe, Fundamentals of Database Systems, Fourth Edition Copyright © 2004 Ramez Elmasri and Shamkant Navathe
  45. 45. OODBMS – a view point Wholesome ‘view’ of the object!! Elmasri/Navathe, Fundamentals of Database Systems, Fourth Edition Copyright © 2004 Ramez Elmasri and Shamkant Navathe
  46. 46. RDMBS – the standard viewSTUDENTStudent# StudentName Address STUDIES1 jane jones 6 The High Street Student# Course#2 brian brown 104 Park Avenue 1 C13 clara clarke 97 Gilmore Street 2 T24 sally smith 68 Lemon Grove 3 T25 tom taylor 53 London Road 4 Q9 5 F COURSE Course# CourseName C1 computing F3 flower arranging Q9 quantum mechanics T2 theology Elmasri/Navathe, Fundamentals of Database Systems, Fourth Edition Copyright © 2004 Ramez Elmasri and Shamkant Navathe
  47. 47. OODBMS – how it looksElmasri/Navathe, Fundamentals of Database Systems, Fourth Edition Copyright © 2004 Ramez Elmasri and Shamkant Navathe
  48. 48. Comparison RDBs vs. OODBs. – RDBs have only one construct i.e. Relation, whereas the type system of OODBs is much richer and complex. – RDBs require primary keys and foreign keys for implementing relationships, OODBs simply don’t. – Optimization of queries in OODBs is much complex than RDBs, but is mainly inspired from the Optimization techniques in RDBs. – OODBs support complex data whereas RDBs don’t. – OODBs are much faster than RDBs but are less mature to handle large volumes of data. – There is more acceptance and domination of RDBs in the market than that for OODBs. Elmasri/Navathe, Fundamentals of Database Systems, Fourth Edition Copyright © 2004 Ramez Elmasri and Shamkant Navathe
  49. 49. RDBMS .. OODBMS SQL is used to ‘query’ a RDBMS. – Operates over a Relational database implemented as a ‘table’ or multiple ‘tables’ and produces a resultant ‘table’ containing rows of entity sets OQL is used to query a ODBMS – OQL statement will query an object database and return a collection-object which contains resultant objects Elmasri/Navathe, Fundamentals of Database Systems, Fourth Edition Copyright © 2004 Ramez Elmasri and Shamkant Navathe
  50. 50. Example of OQL queryThe following is a sample query “what are the names of the black product?”Select distinct p.nameFrom products pWhere p.color = “black” ⇒Valid in both SQL and OQL, but results are different.
  51. 51. Result of the query (SQL) Product no Name Color P1 Ford Mustang BlackOriginal table P2 Toyota Celica Green P3 Mercedes SLK Black- The statement queries a relational database.=> Returns a table with rows. Result Name Ford Mustang Mercedes SLK
  52. 52. Result of the query (OQL)Product no Name ColorP1 Ford Mustang BlackOriginal tableP2 Toyota Celica GreenP3 Mercedes SLK Black- The statement queries a object-oriented database=> Returns a collection of objects. Result String String Ford Mustang Mercedes SLK
  53. 53. 20.7 Summary (1) Object identity: Objects have unique identities that are independent of their attribute values. Type constructors: Complex object structures can be constructed by recursively applying a set of basic constructors, such as tuple, set, list, and bag. Encapsulation of operations: Both the object structure and the operations that can be applied to objects are included in the object class definitions. Elmasri/Navathe, Fundamentals of Database Systems, Fourth Edition Chapter 20-53 Copyright © 2004 Ramez Elmasri and Shamkant Navathe
  54. 54. Summary (2) Programming language compatibility: Both persistent and transient objects are handled uniformly. Objects are made persistent by being attached to a persistent collection. Type hierarchies and inheritance: Object types can be specified by using a type hierarchy, which allows the inheritance of both attributes and methods of previously defined types. Elmasri/Navathe, Fundamentals of Database Systems, Fourth Edition Chapter 20-54 Copyright © 2004 Ramez Elmasri and Shamkant Navathe
  55. 55. Summary (3) Extents: All persistent objects of a particular type can be stored in an extent. Extents corresponding to a type hierarchy have set/subset constraints enforced on them. Support for complex objects: Both structured and unstructured complex objects can be stored and manipulated. Polymorphism and operator overloading: Operations and method names can be overloaded to apply to different object types with different implementations. Elmasri/Navathe, Fundamentals of Database Systems, Fourth Edition Chapter 20-55 Copyright © 2004 Ramez Elmasri and Shamkant Navathe
  56. 56. Summary (4) Versioning: Some OO systems provide support for maintaining several versions of the same object. Comparison : RDBMS & OODBMS Elmasri/Navathe, Fundamentals of Database Systems, Fourth Edition Chapter 20-56 Copyright © 2004 Ramez Elmasri and Shamkant Navathe
  57. 57. Current Status OODB market not growing much per se. Currently around $250M as opposed to the relational DB revenue upwards of $50B. O-O ideas are being used in a large number of applications, without explicitly using the OODB platform to store data. Growth: O-O tools for modeling and analysis, O-O Programming Languages like Java and C++ Compromise Solution Proposed: Object Relational DB Management (Informix Universal Server, Oracle 10i, IBM’s UDB, DB2/II …) Elmasri/Navathe, Fundamentals of Database Systems, Fourth Edition Chapter 20-57 Copyright © 2004 Ramez Elmasri and Shamkant Navathe

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