Object Databases

Baochuan Lu

outline

• Concepts for Object Databases

• Object Database Standards, Languages, and Design

• Object-Relational and Extended-Relational Systems

Introduction

• Traditional data models (relational, network, and hierarchical) is successful in traditional database applications.

• Shortcomings: engineering design, manufacturing, scientific experiments, telecommunications, geographic information systems and multimedia.

• New requirements: complex structure for objects, longer-duration transactions, new data-types for storing images or large textual items, and the need to define non standard application-specific operations.

Introduction

• Key feature of OODB: designer can specify both the structure of complex objects and the operations to them.

• ODMG: a consortium of object-oriented DBMS venders and users.

• Two component of an object: state(value) and behavior(operations).

• Transient objects: exist only during program execution

Introduction

• Persistent object: persist beyond program termination and can be retrieved later.

• Goal of OODB: 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.

Object Identity

• Object Identity (OID): unique, system-generated, for each independent object, immutable (it’s common to use long integers)

•

Object Structure

• A feature of OODB: objects may have an object structure of arbitrary complexity.

• The state of a complex object may be constructed from other objects.

• Object triple representation: (i, c, v) i is a unique OID, c is a type constructor, v is the object state.

Type Constructors

• Basic constructors: atom, tuple, and set.• Others: list, bag, and array.• Represent object as a graph structure: apply the

type constructors recursively.• Identical states: Identical graph in every respect

(OID at every level)• Equal states(shallow equality):

– Same graph structure– Same corresponding atomic values– Internal nodes may have different OIDs

Encapsulation of Operation, Methods

• Main idea: define the behavior of a type of object based on the operations that can be externally applied to objects of that type.

• Method: operation implementation(interface: operation signature)

• Complete encapsulation is stringent.• Visible attributes: may be directly accessed for reading

by external operators, or by high level query language.• hidden attributes: completely encapsulated, can be

accessed only through predefined operators.

Persistence

• Typical mechanism: naming and reachability

• Naming: give an object a unique persistent name (must be unique within a particular database)

• Reachability: works by making the object reachable from some persistent object.

Class Hierarchies and Inheritance

• Attributes and operations are together called functions (attributes resemble functions with zero arguments)

• Type: a type name and a listing of the names of its visible (public) functions:– PERSON: Name, Address, Birthdate, Age, SSN

• Defining a type: – defining all functions– Implementing them as attributes or as methods

extends

• Extends: collections of objects of the same type

• It’s common in database application that each type or subtype will have an extent associated with it, which holds the collection of all persistent object of that type or subtype.

• Persistent collection, transient collection

Complex Objects

• Two main types: structured and unstructured• Structured:

– made up of components– Defined by applying available type constructors recursively

• Unstructured: a data type requires a large amount of storage (image or large textual object)

• ODBMS can be viewed as having a extensible type system.

• DBMS internals provide the underlying storage and retrieval capabilities.

Other Concepts

• Polymorphism: same name bound to different implementations

• Mutiple inheritance

• Selective inheritance

Object Database Standards, Languages, and Design

Introduction

• Why a standard is needed?• Portability: execute an application program on

different systems with minimal modifications to the program.

• Interoperability• ODMG 2.0 standard: object model, object

definition language (ODL), object query language (OQL), and bindings to object-oriented programming languages.

Object Model of ODMG

• The data model upon which ODL and OQL are based.

• Provides: data type, type constructors

• SQL report describes a standard data model for relational database.

Object Model of ODMG

• Object and literal:– Literal has only a value but no object identifier– Object has four characteristics:

• identifier

• Name

• life time (persistent or not)

• Structure (how to construct)

Object Model of ODMG

• Atomic object: not a collection, includes structured objects created using the struct constructor

• Three types of literals: – Atomic– Collection– Structured

Object Model of ODMG

• Atomic literals: correspond to the values of basic data types, predefined

• Collection literals: specify a value that is a collection of objects or values but the collection itself does not have an OID.

• Structured literals: values that are constructed using the tuple constructor (Date, Interval, Time, Timestamp), user defined are created uing the struct keyword in ODL.

Object Model of ODMG

• Dictionary<k, v>: a collection of associations <k, v>, k is a key (a unique search value) associated with a value v.

• Factory object: an object that can be used to generate or create individual objects via its operations.

Object Definition Language (ODL)

• Designed to support the semantic constructs of the ODMG data model.

• Independent of any programming language.• Create object specifications: classes and interfaces• Specify a database schema:

– In ODL

– Use the specific language bindings to specify how ODL constructs can be mapped to constructs in specific programming language.

Object Query Language (OQL)

• Embedded into one of these programming languages

• Return objects that match the type system of that language

• Similar to SQL with additional features (object identity, complex objects, operations, inheritance, polymorphism, relationships)

Object Query Language (OQL)-Entry Points and Iterator

Variables• Entry point: named persistent object (for

many queries, it is the name of the extent of a class).

• Iterator variable: when a collection is referenced in OQL query.– d in departments– departments d – departments as d

Object Query Language (OQL)-Entry Points and Iterator

Variables• Example

SELECT d.dnameFROM d in departmentsWHERE d.college = ‘Engineering’;

Object Query Language (OQL)-Query Results and Path

Expressions• Any persistent object is a query, result is a

reference to that persistent object.• Path expression: once an entry point is specified, it

can be used to specifies a path to related attributes and objects.

• E.g.Q1: departments;Q2: csdepartment.chair;Q2a: csdepartment.chair.rank;Q2b: csdepartment.has_faculty;

Object Query Language (OQL)-Query Results and Path

Expressionsselect f.rankfrom f in csdepartment.has_faculty;select distinct f.rankfrom f in csdepartment.has_faculty;

Object Query Language (OQL)

• OQL query can return a result with a complex structure specified in the query itself by utilizing the struct keyword. E.g.

select struct (name:struct(last_name: s.name.lname, first_name: s.name.fname),

degree:(select struct (deg:d.degree, yr: d.year, college:d.college) from d in s.degrees)from s in csdepartment.chair.advises;

Object Query Language (OQL)-Specifying Views as Named

Queries• define keyword: specify an identifier of the

named query, unique among all named objects, class names, method names, or function names in the scheme.

• A query definition is persistent until it is redefined or deleted.

Object Query Language (OQL)-Specifying Views as Named

Queries• A view can have parameters.

• Example:define has_minors(deptname) asselect sfrom s in studentswhere s.minors_in.dname = deptname;

Object Query Language (OQL)-Extracting Single Elements from

Singleton Collections• element operator: guaranteed to return a single

element from a singleton collection that contains only one element.

• Example:element (select d from d in departments where d.dname = ‘Computer Science’);

Object Query Language (OQL)-Collection Operators

• Aggregate operators: min, max, count, sum, and avg.

• Membership and qualification expressions: returns a boolean type.

• Example:count (s in has_minors(‘Computer Science’));

Object Query Language (OQL)-Ordered (Indexed) Collection

Expressionsfirst (select struct(faculty:f.name.lname,

salary:f.salary) from f in faculty order by f.salary desc);

Object Query Language (OQL)-Grouping Operator

• group by clause: provides explicit reference to the collection of objects within each group or partition.

select struct (deptname, number_of_majors: count (partition))

from s in studentsgroup by deptname: s.majors_in.dname

• A having can be used to filter the partitioned sets (that is select only some of the groups based on group conditions).

Object Database Conceptual Design

• ODB: relationships are handled by OID references to the related objects.

• RDB: relationships among tuples are specified by attributes with matching values (value references).

Object-Relational and Extended-Relational Systems

• ORDBMS: enhancing the capabilities of RDBMS with some of the features in ODBMS.

• Object-oriented features in SQL-99

Any Queries?

• Hint: use OQL.