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In the case of our guide relation, this means that the mediated schema should
contain the relation
Guide(Name, ID, Bio)
. Additionally, there also needs to be
mappings between the source schemas and the mediated schema. This is done
through two sets of views. First, a set of views define an intermediate schema in
terms of the mediated schema. These are called LAV views after the data inte-
gration architecture, where local sources are defined as views over the mediated
schema [
Vijayaraman et al. 1996
]. Continuing with our travel example, the LAV
views are:
I-TravelOn-Guide(Name, ID, Bio) :- Guide(Name, ID, Bio)
I-GoTravel-Guide(Name, Bio):- Guide(Name, ID, Bio)
.
A separate set of views defines the intermediate schema in terms of the sources.
These are called global-as-view (or GAV) mappings after the data integration archi-
tecture, where the global sources are defined as views over the mediated schema
(see [
Lenzerini 2002
] for a discussion of GAV as well as how it relates to LAV).
Our final set of views for our travel example is thus the GAV views:
I-TravelOnGuide(Name, ID, Bio):- TravelOn-Guide(Name, ID),
TravelOn-Bio(ID, Bio)
I-GoTravel-Guide(Name, Bio) :- GoTravel-Guide(Name, Bio)
.
An interesting result of this paper is that the mappings that are created between
the mediated schema and the sources are a very limited form of global-local-as-view
(GLAV) mappings [
Friedman et al. 1999
] (i.e., mappings where either the local or
the global schema can be defined as the head of a view); in particular, the LAV
views each only have one subgoal in them. This is important because the LAV views
require using answering queries using views (see [
Halevy 2001
] for a survey), and
having only one subgoal in the LAV view means that answering queries is very fast.
This is particularly of note since the local sources will be related to each other in this
fashion - regardless of how the mediated schema is created - so this result shows
what we should expect even if the mediated schema is created in some other fashion.
4.3
BAV
Both-as-view (BAV) [
Pidduck et al. 2002
;
McBrien and Poulovassilis 2003
]isa
framework to support schema transformation and integration. Similar to GLAV
mappings [
Friedman et al. 1999
], BAV allows the definition of views between the
mediated schema and the local sources in both direction - it treats both the global
and the local schemas as sources. A key focus of their work is the transformations
that make this possible - how can the mediated schema be related to the source
schemas. They additionally provide a method to update a mediated schema based
on the integration of new source schemas. To do so, they create a mapping that
directly calls for the addition, deletion, and renaming of attributes and relations in
the mediated schema.
