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5 CONCLUSIONS
In previous bucket-based rewriting algorithms, for a
given query Q, view selection is done by unifying Q
with the definition of views. In other words, the
meaning of “relevant to Q” is in terms of unification.
We found that there is another explanation about
“relevant to Q”, i.e., in the presence of domain
semantics. That is, we can remove the irrelevant
views which could not be found in any bucket-based
algorithm. Also, in some cases, we can avoid the
problem of missing relevant views, which occurs in
bucket-based algorithms.
In this paper, we have aimed to solve the
problems of missing query rewritings and redundant
query rewritings in bucket-based rewriting
algorithms so that we can improve the soundness
and completeness of these algorithms. In the
presence of domain semantics in a mediated schema,
we first compute the pseudo residue for each
constraint over the views using the resolution
method. In fact, what we have done is to transfer the
integrity constraints over the relations of the
mediated schema into a rule over a view. As a result,
for a given query, we can determine which view is
irrelevant to the query, in the presence of domain
semantics, by making a comparison between the
pseudo residue of a view and the comparison
expression of the query. The pseudo residues can be
calculated in advanced, which means that the total
increased computation in Step 1 in our algorithm is
only in polynomial size of |D|*|V|, where |D| and |V|
are the number of domain semantics in a mediated
schema and of the views respectively. This process
is useful for query rewriting, which has been shown
by examples in Section 1.
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