CHAPTER 12 [OBJECT DATABASE CONCEPTS]

Naming Mechanism

object assigned a unique name in object base, user finds object by its name

Reachability

object referenced from other persistent objects, object located through references

What are the Type Inheritance rules:

All attributes/operations are inherited Order of supertypes in UNDER clause determines inheritance hierarchy Instance (object) of a subtype can be used in every context in which a supertype instance used Subtype can redefine any function defined in supertype

What is the UDT (user-defined types) used for?

Can be used to create a complex type for an attribute (similar to struct - no operations) Or: can be used to create a type as a basis for a table of objects (similar to class - can have operations)

How to Map an EER Schema to an ODB Schema

Create ODL class for each EER entity type Add relationship properties for each binary relationship Include appropriate operations for each class ODL class that corresponds to a subclass in the EER schema (Inherits type and methods of its superclass in ODL schema)

Reference Type

Create unique object identifiers (OIDs) Can specify system-generated object identifiers Alternatively can use primary key as OID as in traditional relational model

i = o.create_iterator() What does this mean?

Creates an iterator object for the collection To loop over each object in a collection

Collection (or multivalued) type constructor include?

Set (T) Bag (T) List (T) Array(T) Dictionary (K,T) These constructors are considered to be type generators

What does Class specify?

Specifies both state (attributes) and behavior (operations) of an object type Instantiable

What does Interface specify

Specifies only behavior of an object type Typically noninstantiable

Object database features that have been included in SQL:

Type constructors (generators) Mechanism for specifying object identity Encapsulation of operations Provided through user-defined types (UDTs) Inheritance mechanisms

How to create tables based on UDT

UDT must be INSTANTIABLE One or more tables can be created Table inheritance: - UNDER keyword can also be used to specify supertable/subtable inheritance -Objects in subtable must be a subset of the objects in the supertable

Subtype

Useful when creating a new type that is similar but not identical to an already defined type Subtype inherits functions Additional (local or specific) functions in subtype Example: -EMPLOYEE subtype-of PERSON: Salary, Hire_date, Seniority -STUDENT subtype-of PERSON: Major, Gpa

3 types of constructors (type generators):

1. Atom [basic data type (int, string, etc.)] 2. Struct [tuple] 3. Collection (or multivalued)

4. Typical operations that must be defined elsewhere in programming language.

1. Constructor Operation 2. Destructor Operation 3. Modifier Operations 4. Retrieve operation

What is the difference between between Object and Literal?

Object has five aspects: Identifier, name, lifetime, structure, and creation & Literal Value that does not have an object identifier

Extent

A named persistent object to hold collection of all persistent objects for a class

Behavior Inheritance

Also known as IS-A or interface inheritance.

Objects and Literals

Basic building blocks of the object model

What is the philosophical difference between relational model and object model of data?

Behavioral Specification

Encapsulation of operations

Both the object structure and the operations that can be applied to individual objects are included in the class/type definitions

UDT syntax

CREATE TYPE <type name> AS (<component declarations>);

Requirement that all objects be completely encapsulated is too stringent, what is one way to relax this requirement?

Can divide structure of object into visible and hidden attributes

What does the keyword REF specify?

Component attribute of one tuple may be a reference to a tuple of another table

What is the structure of arbitrary complexity?

Contain all necessary information that describes object or literal because in general, in traditional database systems, information about a complex object is often scattered over many relations or records, leading to loss of direct correspondence between a real-world object and its database representation. In ODBs, a complex type may be constructed from other types by nesting of type constructors.

How to map an EER schema to an ODB schema using Categories (union types)

Difficult to map to ODL

What notation applies to the 4 operations to object?

Dot notation

Transient Collection

Exists temporarily during the execution of a program

Another main characteristic of ODBs is that they do NOT allow type hierarchies and inheritance. TRUE OR FALSE

False. Another main characteristic of ODBs is that they do allow type hierarchies and inheritance.

Nesting Type Constructors

Generate complex type form other types

What is an 'Instance Variable' (attributes) in Object DB?

Holds values that define internal state of object

What happens if the keyword INSTANTIABLE is left out?

If keyword INSTANTIABLE is left out, can use UDT only as attribute data type - not as a basis for a table of objects

How does the unique identity implemented?

Implemented via a unique, system-generated object identifier (OID)

What is an example of the Operator Overloading concept?

In some languages, the operator symbol "+" can mean different things when applied to operands (objects) of different types. If the operands of "+" are of type integer, the operation invoked is integer addition. If the operands of "+" are of type floating point, the operation invoked is floating-point addition. If the operands of "+" are of type set, the operation invoked is set union. The compiler can determine which operation to execute based on the types of operands supplied.

What is the use of the extends inheritance?

Inherit both state and behavior strictly among classes

Inheritance

Inheritance allows the definition of new types based on other predefined types, leading to a type (or class) hierarchy.

Collection Objects

Inherits the basic Collection interace

What is Encapsulation related to?

It is related to the concepts of abstract data types

Type Generators

Many different types can be created. Example: set(string), set(integer), and set(Employee) are different types that can be created from the set type constructor.

How to map an EER schema to an ODB schema with an n-ary relationship with degree n>2

Map into a separate class, with appropriate references to each participating class

How to map an EER Schema to an ODB Schema with Weak entity types

Mapped same as regular entity types

Modifier Operations

Modify the state of an object

What are the typical mechanisms for making an object persistent?

Naming and Reachability mechanism

What is the Object identity?

Objects have unique identities that are independent of their attribute values and are generated by the ODB (Object database) system.

What is the one goal of the ODB (object database)?

One goal of an ODB is to maintain a direct correspondence between real-world and database objects so that objects do not lose their integrity and identity and can easily be identified and operated upon

Key

One or more properties whose values are unique for each object in extent of class

What does the Operator Overloading name refer to?

Operation name may be referred to several distinct implementations

Operation signature

Operation name, argument types, return value

What is Operator Overloading ability?

Operation's ability to be applied to different type of objects

Inheritance

Permits specification of new types or classes that inherit much of their structure and/or operation from previously defined types or classes

Encapsulation of operations

Provided through user-defined types (UDTs)

Inheritance mechanisms

Provided using keyword UNDER

CARDINALITY function

Returns the current number of elements in an array

In the Object Database, the operation is defined in what two parts?

Signature (interface) and Implementation (method)

What does keyword 'inverse' consist of?

Single conceptual relationship in inverse directions

What is the Behavior Inheritance specified by?

Specified by the colon (:) notation

Atomic (User-Defined) Objects

Specified using keyword 'class' in ODL

Type constructors (generators)

Specify complex types using UDT

What does the keyword SCOPE specifies

Specify name of table whose tuples referenced

What 2 components does an object database have?

State (value) and Behavior (operations)

Persistent Objects

Stored in database, persist after program termination. The typical mechanisms for making an object persistent are naming and reachability.

Persistent Collection

Stored permanently in the database

How is the concept of encapsulation applied to database objects in ODBs?

by defining the behavior of a class of object based on operations that can be externally applied

Multiple Inheritance

Subtype inherits functions (attributes and operations) of more than one supertype

What does ODL (object definition language) support?

Supports semantic construct of ODMG object model

What does it mean when the OID is immutable?

The OID value of a particular object should not change. This preserves the identity of the real-world object being represented. Also, It is also desirable that each OID be used only once; that is, even if an object is removed from the data- base, its OID should not be assigned to another object.

What is the concept of Encapsulation

The concept of encapsulation is one of the main characteristics of OO languages and systems.

What is the difference between conceptual design of ODB and RDB?

The difference is the handling of: Relationships and Inheritance

NOT FINAL

The keyword NOT FINAL indicates that subtypes can be created for that type

UNDER

The keyword UNDER is used to create a subtype

Collection (or multivalued) type constructor mean?

The set(T), list(T), bag(T), array(T), and dictionary(K,T), allow part of an object or literal value to include a collection of other objects or values when needed. MAKE SURE ALL ELEMENTS IN COLLECTION VALUE MUST BE SAME TYPE. Example: all values in collection of type set(string) must be string values

What is the OID (object identifier)?

The value of an OID may not be visible to the external user but is used internally by the system to identify each object uniquely and to create and manage interobject references. The OID can be assigned to program variables of the appropriate type when needed.

What are the external users of the object aware of?

They are only made aware of the interface of the operation, which defines the name and arguments (parameters) or each operation

What is the concept of Polymorphism of operations?

This concept allows the same opera- tor 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.

What is Polymorphism of Operations also known as?

This is also known as operator overloading

ODMG (Object Data Management Group) model

This is the data model for object definition language (ODL) and object query language (OQL)

Not all objects are meant to be stored permanently in the database, what is the solution?

Transient Objects and Persistant Objects

Most OO database systems allow for the representation of both objects and literals (simple or complex values) TRUE or FALSE

True

Multiple inheritance via extends not permitted in the EXTENDS inheritance? TRUE OR FALSE

True

Object Definition Language (ODL)

Used o define objects types for a particular database application

Constructor Operation

Used to create a new object

Destructor Operation

Used to destroy (delete) an object

Factory Object

Used to generate or create individual objects via its operations

Array Type

Used to specified collections

Is ODL independent of any particular programming language?

Yes

Does an object have a unique identity?

Yes, the unique identity is generally assigned to each independent object stored in the database.

Relationship in Atomic (user-defined) objects

a property that specifies that two objects in the database are related. This is represented by the keyword 'inverse'

What is the use for the dot notation?

builds path expressions

Type

defined by assigning it a type name and list of visible (public) functions (attributes or operations) since attributes resemble functions with zero arguments

Transient Objects

exist in the executing program and disappear once the program terminates

What keyword is specifies the EXTENDS inheritance?

extends

Attributes in Atomic (user-defined) objects

property that describes some aspect of an object

What does Behavior refer to

refers to operations

What does State refer to

refers to properties (attributes)

Collection objects further specialized into:

set, list, bag, array, and dictionary

In SQL, a what is user-defined type and what is the format?

specifies methods (or operations) in addition to the attributes FORMAT: CREATE TYPE <TYPE-NAME> ( <LIST OF COMPONENT ATTRIBUTES AND THEIR TYPES> <DECLARATION OF FUNCTIONS (METHODS)> );

What is this used for "-->"

used for dereferencing

Instantiable

user can create one or more tables based on the UDT

How to reference arrays

using []

How to specify that UDT is instantiable

using the phrase INSTANTIABLE