Chapter 1 - User-defined types and typed tables

Question 1

What are the basic OO features that must be supported in an ORDBMS?

Answer 1

Object identity
classes or types
inheritance
encapsulation
overriding + late binding
complex objects
completeness (for method implementation)
extensibility

Question 2

What are the key features and benefits of USER-defined types?

Answer 2

• extend DBS functionality + flexibility
• strong typing
• information hiding (no impl. details)
  Benefits:
• Reuse: overloading + inheritance
• change: isolation + low impact

Question 3

In which sense do UDTs support strong typing?

Answer 3

• type checking can be enhanced with semantics -> not based just on primitive types.
  Shoe size different of room area -> but both integers

Question 4

How does the cast mechanism of distinct types work?

Answer 4

DT: Renamed type with different semantics (share interval representation)
Cast: allows comparison/ordering based os source type/ create DT values from literals, etc..
* cast functions automatically created (CAST (SOURCE AS DISCTINCT) WITH F)

Question 5

How can we ensure the compatibility between source and distinct type without explicit casting?

Answer 5

Relaxed strong typing for assignments USING CAST FUNCTIONS:
CAST (SRC AS DST) WITH F AS ASSIGNMENT
* comparison requires explicit casts *

Question 6

How to distinct types compare to domains in the relational model?

Answer 6

in rel. model -> domain = type

in SQL -> domain = type + constraint + default + collation (CREATE DOMAIN)

Question 7

What are the intensional and extensional aspects of Structured Types?

Answer 7

• Intention = description = type/class -> UDTs
  - > concerns interface and substitution inheritance (operations)
• Extension = actual things = set of objects -> typed tables
  - > Concerns instantiated objects and inclusion inheritance (structure/state)

Question 8

what are the different ways in which objects (i.e. instances of structured types) may exist?

Answer 8

Object as values: Values of a ST (columns, attr. of other ST, domains, variables, expressions)
-> no identity, no state modifications
Objects as rows: in typed tables (oid column = identity, update statement = modification

Question 9

What are two possible instantiation mechanism of structured types

Answer 9

• System-supplied constructor function type-name ->returns obj with default values
• NEW operator -> calls user-defined constructor

Question 10

How can we access the attributes of an object in the query language?

Answer 10

• dot notation for objects as value (a.b.c.d)
• SELECT or UPDATE of columns for typed tables
• ”->” for referenced stored in tables = DEREF followed by dot

Question 11

How are subtyping and inheritance realized with structured UDTs?

Answer 11

Just like OO languages
CREATE TYPE … UNDER … -> inherits attributes and methods
**substitutability implied

Question 12

What are the equivalent of abstract and final classes in an ORDBMS?

Answer 12

• Abstract= NOT INSTATIABLE (no system-supplied constructor)

* final = not supported -> all DTs are final, all STs are not final.

Question 13

What is the meaning of subtype substitutability and how is it realized in an ORDBMBS?

Answer 13

B < A -> B supports at least the operations defined in A and B can be used everywhere an A is expected.
**obs: invariant return , argument and attribute types

Question 14

How can we test for the type of an object in SQL?

Answer 14

• Determination of dynamic type. IS OF (boolean = instance of)
• subtypes out: ONLY(type) (type == getClass( ))

Question 15

What are typed tables and what characteristics do they provide to objects?

Answer 15

tables whose columns are attributes of an ST and rows are objects with an identity
-> provide identity + persistence + update capability

Question 16

How can we modify the state of objects in typed tables?

Answer 16

modify obj attributes by updating column values

Question 17

How do table hierarchies relate to type hierarchies?

Answer 17

(Table inherits constraints, triggers, etc)

• typed table can be declared under another typed table of a super type
• *but columns only of the super-type table
• > to exclude subtypes-> FROM ONLY table

Question 18

How is the concept of object identity supported in ORDBSs?

Answer 18

(identity->existence != values) (sharing, updates, references….)
Primary keys not enough: modifiable, not uniform across tables, require join to navigate into obj
*Solution: oid column (pk) maintained by system + reference types.

Question 19

What are the variation criteria for object identity in ORDBMSs?

Answer 19

• externall/internal *system/user generated (define identity upon creation)
• Scope of uniquenes (maximal type in table hierarchy)
• reusability (reusing id of deleted obj)

Question 20

What are reference types and different approaches for their representation?

Answer 20

Used to reference instances of STs stored in typed tables

• > user generated (REF USING )
• > System generated (REF is system gen.)
• > Derived from other attrs (REF FROM (…))

Question 21

Give an overview over the general use of reference types in the query language

Answer 21

• Insertion
• > User gen.: Like any other, with costs provided
• > System gen.: retrieved from destination table on a subquery
• Strong typing
• > Comparison/assignment requires some REF type
• > Substitutability supported: REF(sub) compatible with REF(super)

Question 22

What is the requirement on a reference type in order to allow dereferencing it?

Answer 22

• Must be scoped (limited to single table)
  why?:
  -> Dynamic checks (delete, authorization …)
  ->Updates

Question 23

What are path expressions and how do they work when scoped references are used?

Answer 23

Scoped references: When creating table, specify precisely the table in which referenced objects are stored (ref-attribute-name) WITH OPTIONS SCOPE [references are checked]
path expressions:
ref-of-attributte
acesses: a->b.c

Question 24

What is the functionality of the DEREF operator?

Answer 24

Retrieve object (structured type value) from reference

Question 25

What is the relation between reference types and referential constraints?

Answer 25

• > Ref. constraints specify inclusion dependencies (no dereferencing)
• > no strong typing

Question 26

How do the operators ONLY and IS OF work when applied to typed tables?

Answer 26

• > DEREF (obj) IS OF (type)

- > SELECT … FROM ONLY (super)

Question 27

What are the different forms of equality and how are they used in SQL?

Answer 27

• > Shallow-equality of objects = reference attributes compared by identity
• > deep-equality = derefs nested objects, comparing atomic values by deep equality

Question 28

What is the goal of UDT ordering and the different types of comparison supported?

Answer 28

Goal: allow comparison (equality) of objects
types: EQUALS ONLY (=, !=), ORDER FULL (all)

Question 29

What are different approaches for defining ordering fnctions and in which scenarios are they allowed?

Answer 29

STATE: Function automatically created, comparing attributes pair wise (equals) **2 param
RELATIVE: Specify function to compare two objects of the same type and return integer (compare to) **2 parameters
MAP: Map object to predefined type and compare them normally (hash code) **1 param

Question 30

Explain the concept of comparison type.

Answer 30

Nearest supertype for which comparison was defined

comparison = ordering of comparison type

Question 31

What is the goal of USER-defined Casts?

Answer 31

Allow values to be cast to other types
-> from/to UDTs and reference type
CREATE CAST (T1 AS T2) WITH FUNCTION F(T1) [AS ASSIGNMENT]