Object Databases: What can we learn and what should we teach

1
Object Databases What can we learn and what
should we teach?

• M.J.Ridley
• Dept. of ComputingSchool of InformaticsUniversit
  y of Bradford M.J.Ridley_at_brad.ac.uk
  http//www.inf.brad.ac.uk/mick

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Background Context

• Database Systems compulsory second year database
  module on relational theory and RDBMSs, with a
  token historical nod to pre-relational systems.
• Advanced Database Systems (ADS) now in its fifth
  year, compulsory part of some courses and an
  option for others.
• Focused on object and object-relational systems.
  The material in ADS has taken the Object-Oriented
  Database Manifesto and the Third Generation
  Database System Manifesto as its starting point.

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Background Context

• Main focus on contrasting object and
  object-relational approaches supported with O2
  and PostgreSQL.
• Additional reviews of other topics
• Taught by extending the relational approach first
  and then looking at the object approach as an
  alternative.

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Not a systematic study of the rise and fall of
ODBMSs.

• ODMG members
• 02 gone
• Poet merged with Versant
• Gemstone
• Objectivity
• ODMG disbanded Java binding to JDO
• No new books on Object Databases since 2000
• Object Database sections in DB textbooks

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(Object) (Database) Design

• Java and object-oriented methodologies for
  software development and the use of UML as a
  notation.
• Do students come to database design from an
  object-oriented perspective?
• Do they find the object database model close to
  their expectations and a more natural fit?
• Elsewhere students come naturally to want to use
  object-oriented features like inheritance and
  have problems implementing it in systems using
  PostgreSQL.

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(Object) (Database) Design

• Some success
• address has an internal structure and its parts
  should make a whole as part of a larger object
  representing a person.
• Contrasted with columns for st_address and
  town_address.
• list of names for authors of a book.
• stronger, and extensible, typing available in
  object systems.

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(Object) (Database) Design

• Some Failure
• Allocating visible, primary key like, ids to
  objects
• Using these ids to connect objects.
• Seen when apparently object based designs
  suddenly sprout relational features

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Query Languages

• Issues of
• semantic gap between the set-oriented features of
  SQL and the record-at-a-time approach of
  procedural languages.
• relational algebra is an algebra working on
  relations.
• SQL vs RA
• Is this really taken on board by students?

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Query Languages

• How many French Cars are there?
• count(select c from Cars c
• where c.car_manufacturer.country
• "France")
• which returns a number

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Query Languages

• Are there any French cars?
• exists(select c from Cars c
• where c.car_manufacturer.country
• "France")
• which returns a boolean value.

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Query Languages

• select count()
• from cars c, car_manufacturer cm
• where c.man_id cm.id
• and cm.country 'France'
• what's returned by SQL queries
• OQL queries with order by

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Database Systems

• O2
• Exodus and OBST/Stone previously for research and
  projects. Gemstone also considered.
• Postgres leading to PostgreSQL.
• O2 and query language support
• Programming skills requirements.
• Increase in basic start up effort in creating and
  populating a database over the effort for table
  creation and insertion in any relational system.

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Using 02

• OK with small numbers
• Multi-user not multi-developer
• Querying existing schema
• rather than
• Implementing new schema
• Hands on experience of an object system
• (along side hands on with PostgreSQL)

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Future

• O2 no longer developed
• No 02 open source
• What alternative to an object relational system
• Another object system?
• JDO?
• XML DB?

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Conclusions and Questions

• Need for Theory and Practice
• What's the alternative paradigm?
• If it's XML
• How good are the querying facilities?
• How easy it is to create the database structures?
• How easy is XQuery?
• How many features also need knowledge of Xpath?
• How easy is XML Schema?