Databases Reference
In-Depth Information
can be measured in many ways, depending on application domains (e.g., a
number of operations conducted in a minute, hour, or day). Given a certain
time interval, the frequency value (vij) is measured on a scale of ten for the
highest frequency to one for the lowest.
The usage value of each functional dependency can be computed by
summing the frequency values adjusted by the weight of each database
operation. In Exhibit 3, the usage value (UValue
) of functional depen-
dency 1 is computed by summing vc1 * wc, vr1 * wr, vu1*wu, and vd1 * wd
(i.e.,UValue
fd1
wi * vi1 where i = (c, r, u, d)).
The ratio of the usage values for functional dependencies is calculated
by dividing the usage value of the functional dependency by that of the
transitive functional dependency. As shown in Exhibit 3, this ratio is calcu-
lated as follows: U-Ratio = UValue
=
∑
fd1
, where fd1 includes fd2 as a
transitive functional dependency. Because fd2 is part of fd1, whenever fd2
is used, fd1 is automatically used. Thus, the range of U-Ratio is between
zero and one. If the U-Ratio is zero, functional dependency fd2 alone is
never used by an application.
If the U-Ratio value is high (i.e., close to one), the transitive functional
dependency should be normalized. If the U-Ratio is low (i.e., close to zero),
a relation including the transitive functional dependency may well not be
normalized.
/UValue
fd2
fd1
Examples
Exhibit 4 shows a relation, WAREHOUSE_INFO, and identifies functional
dependencies fd1 and fd2. Attributes of MgrName and MgrSalary are func-
tionally dependent on MgrSS# and transitively functionally dependent on
Exhibit 49-4. An example of high FD usage.
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