Database Reference
In-Depth Information
• Spurious tuples: When the SeaView recovery algorithm is
applied to the single-level relations, additional tuples will be
inserted into the original relation. These additional tuples
are called spurious tuples and are a result of repeated joins
between single-level relations.
• Incompleteness: The SeaView decomposition algorithm puts
limitations on the capability of the database. Several relation
instances that have realistic and useful interpretations in real
life cannot be realized in the SeaView model.
• Left outer joins: The SeaView recovery algorithm is based on
the left outer join of relations. It is well known that join is a
high-cost operation and should be avoided as much as possible.
2.4.2 Jajodia-Sandhu Model
The Jajodia-Sandhu model is derived from the SeaView model.
It modifies the algorithm that decomposes a multilevel relation into
single-level fragments and it also modifies the recovery algorithm
that reconstructs the original multilevel relation [25].
In the Jajodia-Sandhu model, the decomposition algorithm uses
only horizontal fragmentation since no vertical fragmentations are
required. This results in improving the recovery algorithm in the
Jajodia-Sandhu model over the recovery algorithm in the SeaView
model because it is possible to reconstruct the multilevel relation
without having to perform join operations; only union operations are
required to reconstruct the multilevel relation.
For example, the relation in Table  2.8 will be decomposed into
two single-level fragments, as shown in Table  2.11. This provides
Table 2.11 Jajodia and Sandhu Decomposition
of Table 2.8 into Two Single-Level Base Relations
A
EMPLOYEE
DEPARTMENT
SALARY
TC
Ahmed U
Accounting U
7,000 U
U
B
EMPLOYEE
DEPARTMENT
SALARY
TC
Ahmed U
Sales S
10,000 S
S
 
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