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 : a collection of associations, 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.dname FROM d in departments WHERE 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 Expressions select f.rank from f in csdepartment.has_faculty; select distinct f.rank from 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) as select s from s in students where 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 Expressions first (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 students group 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.