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
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Background
Requirements
Specifications
Divergence from SQL
Current Situation
Query optimisation
Future Direction
BACKGROUND
Object-Oriented Databases Concepts
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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
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High level language
Efficient
Application independent
 Used to interactively query the database online.
OQL requirements
 An OQL should provide:
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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:
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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:
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Object IDs
Members
Methods
Derived data
Hierarchy
Recursion
O2 Query
 Query language used by O2 DBMS
 2 Modes:interactive & programming
 When interactive, can violate
encapsulation
 High-level SQL-like language
 Generic methods already defined
LIFOO
 Used by O2
 Pure interrogation language
Quantification, aggregation and
Ordering OK
 Does not support recursive queries
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 Functional 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
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Syntax
Use of recursive cursor
 Result of a query is an existing object
 Aggregation and Ordering not supported
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Foreign functions can be used
Data Model is Multi-Model
SQL 3
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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
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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:
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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:
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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:
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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:
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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:
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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.