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Object-Oriented
& Object-Relational DBMSs
CENG 553 Database Management Systems
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Advanced Database Applications
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Computer-Aided Design/Manufacturing (CAD/CAM)
Computer-Aided Software Engineering (CASE)
Network Management Systems
Office Information Systems (OIS) and Multimedia
Systems
Digital Publishing
Geographic Information Systems (GIS)
Interactive and Dynamic Web sites
Other applications with complex and interrelated
objects and procedural data.
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Expected features for new
applications
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Complex objects
Behavioral data
Meta knowledge
Long duration transactions
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Weaknesses of RDBMSs
• Poor representation of “Real World” entities
– Normalization leads to relations that do not correspond
to entities in “real world”.
• Semantic overloading
– Relational model has only one construct for
representing data and data relationships: the relation.
– Relational model is semantically overloaded
• Limited operations
– only a fixed set of operations which cannot be
extended.
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Object-Oriented Concepts
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Abstraction, encapsulation, information hiding.
Objects and attributes.
Object identity.
Methods and messages.
Classes, subclasses, superclasses, and inheritance.
Overloading.
Polymorphism and dynamic binding.
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Database Systems
First Generation DBMS: Network and Hierarchical
– Required complex programs for even simple queries.
– Minimal data independence.
– No widely accepted theoretical foundation.
Second Generation DBMS: Relational DBMS
– Helped overcome these problems.
Third Generation DBMS: OODBMS and ORDBMS.
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Object-Oriented Data Model
No one agreed object data model. One definition:
Object-Oriented Data Model (OODM)
– Data model that captures semantics of objects supported in
object-oriented programming.
Object-Oriented Database (OODB)
– Persistent and sharable collection of objects defined by an
ODM.
Object-Oriented DBMS (OODBMS)
– Manager of an ODB.
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Commercial OODBMSs
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GemStone from Gemstone Systems Inc.,
Objectivity/DB from Objectivity Inc.,
ObjectStore from Progress Software Corp.,
Ontos from Ontos Inc.,
FastObjects from Poet Software Corp.,
Jasmine from Computer Associates/Fujitsu,
Versant from Versant Corp.
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Advantages of OODBMSs
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Enriched Modeling Capabilities.
Removal of Impedance Mismatch.
More Expressive Query Language.
Support for Schema Evolution.
Support for Long Duration Transactions.
Applicability to Advanced Database Applications.
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Disadvantages of OODBMSs
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Lack of Universal Data Model.
Lack of Experience.
Lack of Standards.
Query Optimization compromises Encapsulation.
Object Level Locking may impact Performance.
Complexity.
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Alternative Strategies for Developing an
OODBMS
• Extend existing object-oriented programming language.
– GemStone extended Smalltalk.
• Provide extensible OODBMS library.
– Approach taken by Ontos, Versant, and ObjectStore.
• Embed OODB language constructs in a conventional host
language.
– Approach taken by O2,which has extensions for C++.
• Extend existing database language with object-oriented
capabilities.
– Approach being pursued by RDBMS and OODBMS vendors.
– Ontos and Versant provide a version of OSQL.
• Develop a novel database data model/language.
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Single-Level v. Two-Level Storage Model
• With a traditional DBMS, programmer has to:
– Decide when to read and update objects.
– Write code to translate between application’s object model and the
data model of the DBMS.
– Perform additional type-checking when object is read back from
database, to guarantee object will conform to its original type.
• Conventional DBMSs have two-level storage
model: storage model in memory, and database
storage model on disk.
• In contrast, OODBMS gives illusion of single-level
storage model, with similar representation in both
memory and in database stored on disk.
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Two-Level Storage Model for RDBMS
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Single-Level Storage Model for OODBMS
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Object Data Management Group
(ODMG)
• Established by vendors of OODBMSs to define
standards.
• The ODMG Standard includes :
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Object Data Model (ODM).
Object Definition Language (ODL).
Object Query Language (OQL).
C++, Smalltalk, and Java Language Binding.
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Main Idea: Host Language = Data Language
• Objects in the host language are mapped directly to
database objects
• Some objects in the host program are persistent. Changing
such objects (through an assignment to an instance variable
or with a method application) directly and transparently
affects the corresponding database object
• Accessing an object using its oid causes an “object fault”
similar to pagefaults in operating systems. This
transparently brings the object into the memory and the
program works with it as if it were a regular object
defined, for example, in the host Java program
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SQL Databases vs. ODMG
• In SQL: Host program accesses the database by
sending SQL queries to it (using JDBC, ODBC,
Embedded SQL, etc.)
• In ODMG: Host program works with database
objects directly
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ODMG Data Model
• Distinguishes between objects and pure values (which
are called literals)
• Both can have complex internal structure, but only objects have oids
• Two kinds of classes: “ODMG classes” and “ODMG
interfaces”, similar to Java
– An ODMG interface: only signatures
• does not have its own objects
• cannot inherit from (be a subclass of) an ODMG class – only from
another ODMG interface
– An ODMG class:
• can have attributes, methods with code, own objects
• can inherit from (be a subclass of) other ODMG classes or interfaces
– can have at most one immediate superclass
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ODMG Object Model – Built-in Collections
Set: unordered collections without duplicates.
Bag: unordered collections that do allow duplicates.
List: ordered collections that allow duplicates.
Array: 1D array of dynamically varying length.
Dictionary: unordered sequence of key-value pairs with
no duplicate keys.
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More on the ODMG Data Model
• Can specify keys
• Class extents have their own names – this is what
is used in queries
• Distinguishes between relationships and attributes
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Attribute values are literals
Relationship values are objects
Only binary relationships supported
ODMG relationships have little to do with
relationships in the E-R model
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ODL: ODMG’s Object Definition
Language
• ODL supports semantics constructs of ODMG
• ODL is independent of any programming
language
• ODL is used to create object specification
(classes and interfaces)
• ODL is not used for database manipulation
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ODL Examples (1)
A Very Simple Class
• A very simple, straightforward class
definition :
class Degree {
attribute string college;
attribute string degree;
attribute string year;
};
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ODL Examples (2)
A Class With Key and Extent
• A class definition with “extent”, “key”, and more
elaborate attributes; still relatively straightforward
class Person (extent persons key ssn) {
attribute struct Pname {string fname …} name;
attribute string ssn;
attribute date birthdate;
…
short age();
}
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ODL Examples (3)
A Class With Relationships
• Note extends (inheritance) relationship
• Also note “inverse” relationship
Class Faculty extends Person (extent faculty) {
attribute string rank;
attribute float salary;
attribute string phone;
…
relationship Dept works_in inverse Dept::has_faculty;
relationship set<GradStu> advises inverse GradStu::advisor;
void give_raise (in float raise);
void promote (in string new_rank);
};
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Referential Integrity
class STUDENT extends PERSON {
( extent StudentExt )
attribute Set<String> Major;
relationship Set<COURSE> Enrolled;
inverse COURSE::Enrollment;
}
class COURSE: Object {
( extent CourseExt )
attribute Integer CrsCode;
attribute String Department;
relationship Set<STUDENT> Enrollment;
inverse STUDENT::Enrolled;
}
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Object Query Language (OQL)
• OQL is DMG’s query language
• Provides declarative access to object database using SQLlike syntax.
• Does not provide explicit update operators - leaves this to
operations defined on object types.
• Can be used as a standalone language and as a language
embedded in another language, for which an ODMG
binding is defined (Smalltalk, C++, and Java).
• Embedded OQL statements return objects that are
compatible with the type system of the host language
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Object Query Language (OQL)
• An OQL query is a function that delivers an object whose
type may be inferred from operator contributing to query
expression.
• Query definition expressions is of form:
DEFINE Q as e
• Defines query with name Q given query expression e.
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Example OQL: Extents & Traversal Paths
Get set of all faculty (with identity)
faculty
Get set of all enrollments(with identity)
CourseExt.Enrollment
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Example schema
class Branch (extent branchOffices key branchNo)
{
attribute string branchNo;
….
relationship Manager ManagedBy
inverse Manager::Manages;
void takeOnPropertyForRent(in string propertyNo);
}
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Example (cont.)
class Person {
attribute struct Pname {string fName, string lName}
name;
}
Class Staff extends Person
(extent staff
key staffNo)
{
attribute staffNo;
attribute date DOB;
….
short getAge();
}
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Example (cont.)
class Manager extends Staff
(extent managers)
{
relationship Branch Manages
inverse Branch::ManagedBy;
}
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Example OQL: Extents & Traversal Paths
Find all branches in London
DEFINE londonBranches AS
SELECT b.branchNo
FROM b IN branchOffices
WHERE b.address.city = “London”;
This returns a literal of type bag<string>.
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Example OQL: Extents & Traversal Paths
Find all staff who work at London branches.
londonBranches.Has
This returns set<SalesStaff>.
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Example OQL: Use of structures
Get structured set (without identity) containing name,
sex, and age of all staff who live in London.
SELECT struct (lName:s.name.lName, sex:s.sex,
age:s.age)
FROM s IN Staff
WHERE s.WorksAt.address.city = “London”
This returns a literal of type set<struct>.
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Example OQL: Use of structures
Get structured set (without identity) containing
branch number and set of all Assistants at branches in
London.
SELECT struct (branchNo:x.branchNo, assistants:
(SELECT y FROM y IN x.WorksAt
WHERE y.position = “Assistant”))
FROM x IN (SELECT b FROM b IN branchOffices
WHERE b.address.city = “London”)
This returns a literal of type set<struct>.
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OQL - Creating Objects
A type name constructor is used to create an object
with identity.
Manager(staffNo: “SL21”,
fName: “John”, lName: “White”,
address: “19 Taylor St, London”,
position: “Manager”, sex: “M”,
DOB: date“1945-10-01”, salary: 30000)
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ORDBMS
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Merging Relational and Object
Models
• Object-oriented models support interesting
data types --- not just flat files.
– Maps, multimedia, etc.
• The relational model supports very-highlevel queries.
• Object-relational databases are an attempt to
get the best of both.
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Evolution of DBMS’s
• Object-oriented DBMS’s failed because
they did not offer the efficiencies of wellentrenched relational DBMS’s.
• Object-relational extensions to relational
DBMS’s capture much of the advantages of
OO, yet retain the relation as the
fundamental abstraction.
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ORDBMSs
• Vendors of RDBMSs conscious of threat and
promise of OODBMS.
• Agree that RDBMSs not currently suited to advanced
database applications, and added functionality is
required.
• Can remedy shortcomings of relational model by
extending model with OO features.
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ORDBMSs - Features
• OO features being added include:
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user-extensible types,
encapsulation,
inheritance,
polymorphism,
dynamic binding of methods,
complex objects including non-1NF objects,
object identity.
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Stonebraker’s View
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Objects in SQL:1999
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Object-relational extension of SQL-92
Includes the legacy relational model
SQL:1999 database = a finite set of relations
relation = a set of tuples (extends legacy relations)
OR
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a set of objects (completely new)
object = (oid, tuple-value)
tuple = tuple-value
tuple-value = [Attr1: v1, …, Attrn: vn]
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SQL:1999 Tuple Values
• Tuple value: [Attr1: v1, …, Attrn: vn]
– Attri are all distinct attributes
– Each vi is one of these:
• Primitive value: a constant of type CHAR(…),
INTEGER, FLOAT, etc.
• Reference value: an object Id
• Another tuple value
• A collection value
Only the ARRAY construct is – a fixed size array.
SETOF and LISTOF are not supported.
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Row Types
• The same as the original (legacy) relational tuple type.
However:
– Row types can now be the types of the individual attributes in
a tuple
CREATE TABLE
PERSON (
Name CHAR(20),
Address ROW(Number INTEGER, Street CHAR(20), ZIP CHAR(5))
)
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Row Types (Contd.)
• Use path expressions to refer to the components of row types:
SELECT P.Name
FROM PERSON P
WHERE P.Address.ZIP = ‘11794’
• Update operations:
INSERT INTO PERSON(Name, Address)
VALUES (‘John Doe’, ROW(666, ‘Hollow Rd.’, ‘66666’))
UPDATE PERSON
SET Address.ZIP = ‘66666’
WHERE Address.ZIP = ‘55555’
UPDATE PERSON
SET Address = ROW(21, ‘Main St’, ‘12345’)
WHERE Address = ROW(123, ‘Maple Dr.’, ‘54321’) AND Name = ‘J. Public’
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User Defined Types (UDT)
• UDTs allow specification of complex objects/tuples,
methods, and their implementation
• Like ROW types, UDTs can be types of individual
attributes in tuples
• UDTs can be much more complex than ROW types
(even disregarding the methods): the components of
UDTs do not need to be elementary types
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A UDT Example
CREATE TYPE PersonType AS (
Name CHAR(20),
Address ROW(Number INTEGER, Street CHAR(20), ZIP CHAR(5))
);
CREATE TYPE StudentType UNDER PersonType AS (
Id INTEGER,
Status CHAR(2)
)
METHOD award_degree() RETURNS BOOLEAN;
CREATE METHOD award_degree() FOR StudentType
LANGUAGE C
EXTERNAL NAME ‘file:/home/admin/award_degree’;
File that holds the binary code
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Using UDTs in CREATE TABLE
• As an attribute type:
CREATE TABLE TRANSCRIPT (
Student StudentType,
CrsCode CHAR(6),
Semester CHAR(6),
Grade CHAR(1)
)
A previously defined UDT
• As a table type:
CREATE TABLE STUDENT OF StudentType;
Such a table is called typed table.
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Objects
• Only typed tables contain objects (i.e. tuples with oids)
• Compare:
CREATE TABLE STUDENT OF StudentType;
and
CREATE TABLE STUDENT1 (
Name CHAR(20),
Address ROW(Number INTEGER, Street CHAR(20), ZIP CHAR(5)),
Id
INTEGER,
Status CHAR(2)
)
• Both contain tuples of exactly the same structure
• Only the tuples in STUDENT – not STUDENT1 – have oids.
• This disparity is motivated by the need to stay backward
compatible with SQL-92.
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Querying UDTs
• Nothing special – just use path expressions
SELECT T.Student.Name, T.Grade
FROM
TRANSCRIPT T
WHERE T.Student.Address.Street = ‘Main St.’
Note: T.Student has the type StudentType. The attribute Name is
not declared explicitly in StudentType, but is inherited from
PersonType.
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Updating User-Defined Types
• Inserting a record into TRANSCRIPT:
INSERT INTO TRANSCRIPT(Student,Course,Semester,Grade)
VALUES (????, ‘CS308’, ‘2000’, ‘A’)
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The type of the Student attribute is StudentType. How
does one insert a value of this type (in place of ????)?
– Further complication: the UDT StudentType is
encapsulated, i.e., it is accessible only through public
methods, which we did not define
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Do it through the observer and mutator methods
provided by the DBMS automatically
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Observer Methods
• For each attribute A of type T in a UDT, an SQL:1999 DBMS is supposed to
supply an observer method, A: ( )  T, which returns the value of A (the notation
“( )” means that the method takes no arguments)
• Observer methods for StudentType:
• Id: ( )  INTEGER
• Name: ( )  CHAR(20)
• Status: ( )  CHAR(2)
• Address: ( )  ROW(INTEGER, CHAR(20), CHAR(5))
• For example, in
SELECT T.Student.Name, T.Grade
FROM
TRANSCRIPT T
WHERE T.Student.Address.Street = ‘Main St.’
Name and Address are observer methods, since T.Student is of type StudentType
Note: Grade is not an observer, because TRANSCRIPT is not part of a UDT
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Mutator Methods
• An SQL:1999 DBMS is supposed to supply, for each attribute A
of type T in a UDT U, a mutator method
A: T  U
For any object o of type U, it takes a value t of type T
and replaces the old value of o.A with t; it returns the
new value of the object. Thus, o.A(t) is an object of type U
• Mutators for StudentType:
• Id: INTEGER  StudentType
• Name: CHAR(20)  StudentType
• Address: ROW(INTEGER, CHAR(20), CHAR(5))  StudentType
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Example: Inserting a UDT Value
INSERT INTO TRANSCRIPT(Student,Course,Semester,Grade)
VALUES (
NEW StudentType( ) .Id(111111111) .Status(‘G5’) .Name(‘Joe Public’)
.Address(ROW(123,’Main St.’, ‘54321’)) ,
‘CS532’,
‘S2002’,
‘A’
)
Add a value
for Id
Create a blank
StudentType object
Add a value for the
Address attribute
Add a value
for Status
‘CS532’, ‘S2002’, ‘A’ are primitive values for the attributes Course, Semester, Grade
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Example: Changing a UDT Value
UPDATE TRANSCRIPT
SET Student = Student.Address(ROW(21,’Maple St.’,’12345’)).Name(‘John Smith’),
Grade = ‘B’
Change Name
Change Address
WHERE Student.Id = 111111111 AND CrsCode = ‘CS532’ AND Semester = ‘S2002’
• Mutators are used to change the values of the attributes Address
and Name
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Referencing Objects
• Consider again
CREATE TABLE TRANSCRIPT (
Student StudentType,
CrsCode CHAR(6),
Semester CHAR(6),
Grade CHAR(1)
)
• Problem: TRANSCRIPT records for the same student refer to distinct
values of type StudentType (even though the contents of these
values may be the same) – a maintenance/consistency problem
• Solution: use self-referencing column
– Bad design, which distinguishes objects from their references
– Not truly object-oriented
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Self-Referencing Column
• Every typed table has a self-referencing column
– Normally invisible
– Contains explicit object Id for each tuple in the table
– Can be given an explicit name – the only way to enable
referencing of objects
CREATE TABLE STUDENT2 OF StudentType
REF IS stud_oid;
Self-referencing column
Self-referencing columns can be used in queries just like regular columns
Their values cannot be changed, however
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Reference Types and Self-Referencing Columns
• To reference objects, use self-referencing columns + reference
types: REF(some-UDT)
CREATE TABLE TRANSCRIPT1 (
Student REF(StudentType) SCOPE STUDENT2,
CrsCode CHAR(6),
Semester CHAR(6),
Grade CHAR(1)
Reference type
)
Typed table where the
values are drawn from
• Two issues:
• How does one query the attributes of a reference type
• How does one provide values for the attributes of type REF(…)
– Remember: you can’t manufacture these values out of thin air – they are oids!
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Querying Reference Types
• Recall:
Student REF(StudentType) SCOPE STUDENT2
in
TRANSCRIPT1.
How does one access, for example, student names?
• SQL:1999 has the same misfeature as C/C++ has (and which Java and
OQL do not have): it distinguishes between objects and references to
objects. To pass through a boundary of REF(…) use “” instead of “.”
SELECT T.StudentName, T.Grade
FROM TRANSCRIPT1 T
WHERE
T.StudentAddress.Street = “Main St.”
Not crossing REF(…)
boundary, use “.”
Crossing REF(…)
boundary, use 
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Inserting REF Values
• How does one give values to REF attributes, like Student in
TRANSCRIPT1?
• Use explicit self-referencing columns, like stud_oid in STUDENT2
• Example: Creating a TRANSCRIPT1 record whose Student attribute has
an object reference to an object in STUDENT2:
INSERT INTO TRANSCRIPT1(Student,Course,Semester,Grade)
SELECT S.stud_oid, ‘HIS666’, ‘F1462’, ‘D’
FROM STUDENT2 S
WHERE S.Id = ‘111111111’
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Explicit self-referential
column of STUDENT2
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Modifications to support ORDBMS
• Parsing
– Type-checking for methods pretty complex.
• Query Rewriting
– Often useful to turn path exprs into joins!
• Optimization
– New algebra operators needed for complex types.
• Must know how to integrate them into optimization.
– WHERE clause exprs can be expensive!
• Selection pushdown may be a bad idea.
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Modifications (Contd.)
• Execution
– New algebra operators for complex types.
– OID generation & reference handling.
– Dynamic linking.
– Support “untrusted” methods.
– Support objects bigger than 1 page.
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Modifications (Contd.)
• Access Methods
– Indexes on methods, not just columns.
– Need indexes for new WHERE clause exprs
(not just <, >, =)!
• Data Layout
– Clustering of nested objects.
– Chunking of arrays.
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OO/OR-DBMS Summary
• Traditional SQL is too limited for new apps.
• OODBMS: Persistent OO programming.
– Difficult to use, no query language.
• ORDBMS: Best (?) of both worlds:
– Catching on in industry and applications.
– Pretty easy for SQL folks to pick up.
– Still has growing pains (SQL-3 standard still a
moving target).
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Summary (Contd.)
• ORDBMS offers many new features.
– But not clear how to use them!
– Schema design techniques not well understood
– Query processing techniques still in research
phase.
• A moving target for OR DBA’s!
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