Transcript OODBMS and ORDBMS - UBC Department of Geography

OODBMS and ORDBMS
Background
• Object-oriented software, based on the
principles of user-defined datatypes, along
with inheritance and polymorphism, is
firmly established in the software industry
(consider C++, Java, Visual Basic, C#,
etc.)
• However, the popularity of OO software
has no relation with the popularity, or not,
of OODBMS.
Background
• The debate between relational versus oo
within the DB community parallels the
debate between vector and raster in the
GIS community during the 70’s and 80’s.
• Market adoption of OODBMS has been
limited. This has implications:
– Reduced financial and engineering resources
to performance-tune products.
– SQL has retained its dominance as the lingua
franca of the database world.
Background
• Relational databases that incorporate
ADTs and other principles of oo design are
called object relational DBS (OR-DBMS).
• These databases have taken the steam
out of the OO engine, and appear to have
the upper hand in ‘modern’ DMBS.
Background
• Recall that RDBMS represents themes through
relations (tables).
– A geographic object is one tuple (row) of such a
relation.
– Each column is an attribute.
• Attributes have alphanumeric types (e.g.,
character, string, time, date, real, logical).
BLOBS are a recent addition.
• Relationships are not explicit, but implied by
values in specific fields (e.g., foreign keys).
RDBMS?
• To incorporate spatial concepts into a
RDBMS requires considerable reorganization of the RDBMS database
structure, which violates the data
independence principle (you need to know
far more about a theme in order to
determine what type of spatial entity it is,
and what type of operations are
permissible).
RDBMS
• Performance suffers greatly as a result of
the significant modifications required to
incorporate space.
• Topology is not implicit.
• As a result, most systems were based on
a loose coupling approach (e.g., Arc /
Info).
OO DBMS
• OO systems were first developed in the
1980s, when oo programming languages
such as Simula and Smalltalk first
emerged.
• The initial reaction was that, within a few
years, all databases would be oo
databases. Such predictions were
obviously way off the mark.
OO Advantages
• OO principles brought to databases:
– New modeling power
– Extensibility of systems
– Code reuse
– Easy maintenance of programs
• However, it is only recently that OO
programming has itself really taken off.
OODBMS Terminology
• Object Identity: An object is denoted in a
unique way in the database system, using
an identifier or OID (object identifier) that
the objects keeps during its entire lifetime,
independently of its attribute values.
OODBMS Terminology
• Type: the type of an object corresponds to its
structure and to the operations that can be
performed on it.
– Atomic types, such as strings and integer
– Complex types are defined using ‘constructors’
(tuples or set constructors).
• An object (o) can have as an atomic value not
only a string or real but also the OID of another
object o’. Object o is said to consist of object o’
OODBMS Terminology
• Objects with the same type are grouped
into classes.
• Classes encompass objects having the
same structure and behaviour, which is
expressed by the set of methods or
operations applicable to the objects they
contain.
• Encapsulation: A class corresponds to the
implementation of an abstract data type
(ADT).
– Encapsulation means that neither the
structure of the ADT nor its implementation is
visible from the outside world.
– Programmers will know how the ADT is
implemented, whereas end users only know
the specification.
OODBMS Terminology
• ADTs: Abstract data types – combinations of
data types (e.g., polygon) and their operations
(e.g., PolygonArea [an operation that would
compute the area of a polygon]). Note that the
DBMS need not have any knowledge on the
ADT implementation (i.e., the code
corresponding to the operations).
• ADTs are encapsulated in the sense that they
are accessible only through the operations that
are defined on them.
• ADTs have been ‘regularized’ in the sense
that the geographic dimension (e.g.,
points, line segments [nodes only have
two arcs], complex polylines [nodes can
have two or more connected arcs],
polygons) is used to clearly distinguish
between them.
OODBMS Terminology
• Inheritance: Subclasses are defined from an existing
superclass with a refinement of its structure.
– Subclasses inherit methods and structures from the superclass
(e.g., all methods defined for the superclass can be used in the
subclass).
– However, a subclass can redefine a method defined in the
superclass.
– A subclass can also have methods defined only for that
subclass. This leads to polymorphism.
• The subclass is a specialization of the superclass, and
the superclass is a generalization of the subclass.
• A subclass can inherit from many superclasses.
• Most OO DBMS use OQL rather than SQL as
their query language.
• OQL is aware of the complex structures present
in the OODBMS, and can return complex
results.
• SQL is used as the DDL, DQL and DML (data
definition, data query, and data manipulation
language).
• In OODBMS, the primary means of developing
the database is through the native OO language.
OR DBMS
• PostgresSQL is the best known OR DBMS
• It allows the user to extend the system by
incorporating new data types—a user can
integrate geometric types and operations,
as well as spatial indexing.
• ORDBMS incorporate ADTs, but still use
tables.
• OR SQL contains extensions to standard
SQL than enable access to ADTs.
• Software such as ESRI’s SDE is based on
an ORDBMS approach.