Basic Concept of Multilevel Database Security - Multilevel Security for Relational Databases - page 25

There are two algorithms that are used in the implementation of
the SeaView model:

• The decomposition algorithm divides the multilevel relation
into single-level fragments.
• The recovery algorithm reconstructs the original multilevel
relation from the fragments.

In the SeaView model, the decomposition of the multilevel relations
into single-level ones is performed by applying two different types of
fragmentation: horizontal and vertical. Thus, the multilevel relation in
Table 2.8 will be stored as five single-level fragments (one primary key
group relation and four attribute group relations), as shown in Table 2.10.
The SeaView model has many problems due to the decomposition
and recovery algorithms. These problems will be mentioned as follows:

• Repeated joins: Due to the vertical fragmentations that
are used in the SeaView model, the query that involves
multiple attributes will use a lot of repeated left outer joins
between the several single-level relations to get the result.

Table 2.10 SeaView Decomposition of
Table 2.8 into Five Single-Level Base Relations
A

EMPLOYEE
Ahmed U

B

EMPLOYEE DEPARTMENT

Ahmed U Accounting U

C

EMPLOYEE SALARY

Ahmed U 7,000 U

D

EMPLOYEE DEPARTMENT

Ahmed U Sales S

E

EMPLOYEE SALARY

Ahmed U 10,000 S