OBJECT ORIENTED QUERY LANGUAGES

Object Oriented Database Seminar

Omar Alasam

Julien Aubry

Julien Fontaine

Bénédicte Gruss

School of Computing

Beng CNDS 4 Napier University

PLAN

Background Requirements Specifications Divergence from SQL Current Situation Query optimisation Future Direction

BACKGROUND

Object-Oriented Databases Concepts

Objects and Identity Complex objects Encapsulation Classes Inheritance Overloading, Overriding and late binding

BACKGROUND 2

Query Languages

SQL took 15 years to become the standard of RDBMS Programming languages were first used to query

OODB Users demand on a SQL-like query facility for OODB. No standard yet accepted Today, each OODBMS has its own query language

implemented.

OOQL Requirements

The Object-Oriented Database System Manifesto, 1989 High level language Efficient Application independent

Used to interactively query the database on-line.

OQL requirements

An OQL should provide: Descriptiveness Closure Completeness Expressive power Extensibility orthogonality

Query Language issues

Should the query language be integreted in the programming language?

How to access the data: Simple data Complex data

What is a result to a query ?

Query language issues 2

Possibility to make join request

Management of sets of objects

Equal access to attributes and methods

Consequences of class hierarchy

SQL versus OQL

New concepts have to be added to SQL: Object IDs Members Methods Derived data Hierarchy Recursion

O2 Query

Query language used by O2 DBMS2 Modes:interactive & programmingWhen interactive, can violate

encapsulationHigh-level SQL-like languageGeneric methods already defined

LIFOO

Used by O2Pure interrogation language

Quantification, aggregation and Ordering OK

Does not support recursive queriesFunctional language (handles

functional syntax ; min, max..)

Gemstone

Result of a query: set of class identical as the queried class

Queries: Boolean operations combined to path expression:

Tasks select: {:t | (t.man_years > 20 & (t.leader.specialization = ‘DB’)}

OSQL

Used by IRIS Very close to SQL

Syntax Use of recursive cursor

Result of a query is an existing object Aggregation and Ordering not supported

Foreign functions can be used Data Model is Multi-Model

SQL 3

Object-Oriented version of the famous QL Use of ADT so as to describe classes Encapsulation assured thanks to flags Use of constructors, destructors and actors Compatibility with SQL 92

ADT:Abstract Data Type

OQL

Defined by ODMG OQL adopts the type system of the

application Functional Query Language Orthogonal Query Language

Query Optimisation

Introduction: Difficulties of OODBMS Queries. Optimisation Issues

Additional Data Types: definition of new types and classes through inheritance can both assist and prevent optimisation of queries

Complex Objects: The path expressions and query closure of OODBMS query languages complicate the processing of queries in several ways

 Methods and Encapsulation: creation of new objects, object identities (OIDs).

Cost Determination of evaluating methods.

Optimisation Issues Cont.

 Algebras and Calculi Optimisation: The lack of a standard,  Algebra is variable based. Algebraic optimisation has the advantage that a query can be

transformed using well-defined operators.

 Extensible Optimisers:  Ability to change with time through ``Optimiser Generators'':

builds optimisers based on information on the data model, query language, algebra, calculi and cost model. 

The Query Processing Methodology:

Scope of Future Work

A Standard Extensible Optimiser Framework.

Better Cost Models. Better Algebras and Rewrite Systems. Eliminating Method Evaluation. Indexing of Path Expressions. Pre-computation and Caching.

Solutions and Evaluation:

Simple evaluation, dynamic programming methods, Evolutionary Algorithms and randomised search.

Final words on Query Optimisation: It is hard to provide a scheme for evaluating the

success of a research into query optimisation, the only real criterion being faster average execution time of queries.

OOQL Future Directions

 Innovation in the area of OODBMS query languages !

 Research in OOQL: Super OODBMS Query Language: Polishing ODMG-93 OQL Connection to Rule Systems End-users Queries Graphical Query Construction

OOQL Future Directions 2

Future Work Scope and Expectations from: ODMG-93 OQL and Rule Systems. Implementation before evaluation. Good end-user OODBMS query mechanism

driven by strong industrial backing.

Conclusion

ODMG defined a standard with OQL OQL is competing with SQL 3 Vendors also have their own OOQL The popularity of OODB depends a lot on

the query language that will be adopted.