Transformations of Generalization Hierarchies
Outline
Methods
- Superset method
- Move all attributes to the top level class
- Subset method
- Move all attributes to the leaf level classes
- Association method
- Replace generalization with association
- Hybrid method
- A mixture of superset, subset, and association methods
Transformations of Generalization Hierarchies
Outline
Superset method
- Move the attributes from subclasses to a superclass
- The attributes advisor from a class POSTGRADUATE STUDENT and coordinator from a class UNDERGRADUATE STUDENT are moved to a superclass STUDENT
Superset method
- Eliminate subclasses
- The classes POSTGRADUATE STUDENT and UNDERGRADUATE STUDENT are removed
- An attribute degree is added to a class STUDENT
Superset method
- Final design
- The attributes advisor and coordinator are optional in a class STUDENT becasue only some students have a coordinator while the others have an advisor
- XOR above a dashed line means that each student is linked either to a coordinator or to an advisor and not to both of them
Superset method
- Performance related observations
- Superset method is beneficial when the majority of queries access the attributes from many different levels of generalization hierarchy and when aggregations over different subclasses are performed
- Superset method is opposite to horizontal and vertical decompositions
- Superset method increases the total number of objects to be traversed and a size of each object
Transformations of Generalization Hierarchies
Outline
Subset method
- Move the attributes down to the subclasses
- The attributes s# and sname are moved from a class STUDENT to both classes POSTGRADUATE STUDENT and UNDERGRADUATE STUDENT
Subset method
- Eliminate the superclass
- A class STUDENT is deleted and an association Is-member-of is replicated to connect a class DEPARTMENT with both classes POSTGRADUATE STUDENT and UNDERGRADUATE STUDENT
Subset method
- Final design
Subset method
- Performance related observations
- Subset method is beneficial when the majority of queries access individually the lowest level classes in a generalization hierarchy
- Subset method is equivalent to vertical decomposition of classes (horizontal decompositions of relational tables)
- Subset method decreases the total number of objects to be traversed and it increases a size of each object in a class
- Subset method increases a number of joins when the majority of queries accesses the attributes from many different classes
Transformations of Generalization Hierarchies
Outline
Association method
- Copy an identifer to the subclasses
- An attribute s# is copied to both subclases POSTGRADUATE STUDENT and UNDERGRADUATE STUDENT
Association method
- Replace generalization with one-to-one associations
- Semantics of the classes POSTGRADUATE STUDENT and UNDERGRADUATE STUDENT is changed and a generalization is replaced with one-to-one associations
Association method
- Final design
- A class POSTGRADUATE STUDENT represents now a part of a description of postgraduate students; a class UNDEGRADUATE STUDENT represents now a part of a description of undergraduate students
Association method
- Another way is to use superset method, apply a horizontal decomposition, and remove categorization attribute
- After application of superset method a class STUDENT is decomposed horizontally and an attribute degree is removed.
Association method
- Performance related observations
- Association method is beneficial when the majority of queries access individually the classes in a generalization hierarchy
- Association method is equivalent to horizontal decomposition of classes (vertical decompositions of relational tables)
- Assocition method decreases a size of each object in a class
- Associations method increases a number of joins when the majority of queries accesses the attributes from many different classes
Transformations of Generalization Hierarchies
Outline
Hybrid method
- Original generalization hierarchy
Hybrid method
- Application of subset method
- An attribute registration is moved to both subclasses LAND VEHICLE and WATER VEHICLE
Hybrid method
- Application of superset method
- The attributes max-passengers and capacity are moved to a superclass LAND VEHICLE
Hybrid method
- Transformation of (p,e) generalization into (t,e) generalization
- A p-e generalization SHIP ISA WATER VEHICLE is replaced with a t-e generalization SHIP ISA WATER VEHICLE and OTHER (WATER VEHICLE) ISA WATER VEHICLE
Hybrid method
- Application of association method
- Association method is applied to t-e generalization
- Semantics of classes SHIP and OTHER is changed such that a class SHIP represents a part of a description of ship and a class OTHER represents a part of a description of other water vehicle
Hybrid method
- Final design
- A description of water vehicle consists of either a description of ship or a description of other water vehicle