Databases Reference
In-Depth Information
6
Generic Schema Merging
6.1
Buneman, Davidson, and Kosky
Buneman et al.
[
1992
] delve into some of the theory of schema merging. In particu-
lar, they start once again with the assumption that elements are matched by name -
i.e., they have avoided the matching problem entirely. The goal is to think of this
from a lattice perspective (much like the one later used by FCA Merge) and describe
two different theoretical perspectives: either the least upper bound (i.e., take every-
thing that is available in either schema, which is rather like taking the union) or the
greatest lower bound (i.e., take everything that is available in both schemas, which
is rather like taking the intersection).
They, like most works here, focus on the least upper bound. Once having decided
that the least upper bound is the correct semantics, the question is what kind of
conflicts should be resolved. They use a very basic meta-model to allow them to con-
centrate on some very fundamental conflicts. In particular, their meta-model allows
for elements which have only names as attributes. Their meta-model allows for two
types of edges: is-a and has-a. They represent the fact that an element
r
Has-a ele-
ment
x
of type
y
by an arrow from
r
to
y
with the label
x
. They do not consider
constraints on the instances. Given these limited types of edges, they can focus on
what would happen if two elements are combined resulting in the merged element
having two types. For example, Fig.
8.3
a says that element
R
has a
Q
of type
P
and
S
. Naturally, this does not make sense. Hence, their solution is to say that there
should be a new type, and that both of the original types inherit from this type, as
showninFig.
8.3
b.
This kind of work shows the fundamental issues that have to be explored to merge
schemas regardless of application or data model.
6.2
Model Management
Pottinger and Bernstein
[
2003
] and Rondo [
Melnik et al. 2003
] both describe
merge operators for Model Management [
Bernstein et al. 2000
]. The goal of Model
a
b
P
R
S
Q
P
Q
R
Q
S
T
Fig. 8.3
Buneman et al.
[
1992
] show that one conflict that occurs during the merging of schemas
is that if there are two elements in the resulting merge that have different types (
a
), then a new type
can be created which inherits from both original types (
b
)
