By Philippe Cudre-Mauroux
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Pred (sa) = σAi This can be captured by calculating a value FVσi for every attribute Ai ∈ sa as follows: if Ai ∈ sa and targetµ (Ai ) = ⊥ then FVσi = 1 else FVσi = selAi , where selAi is the selectivity of the predicated predAi . e. predicates selecting a small proportion of the database. In that way, dropping highly selective, critical attributes leads to lower values of FVσi . 43 Semantic Gossiping 2. Not all attributes in pa are preserved. Therefore, some of the results may be incomplete or even erroneous (due to the loss of key attributes, for instance). 4, are syntactic in nature and relate to the completeness of the reformulations that involve local mappings. The notion of syntactic similarity is introduced to quantify the extent to which a query reformulation is complete. 5, relate to the degree of agreement that can be achieved among different peers upon sets of attributes. Degrees of agreement can be evaluated using feedback mechanisms. Below, we introduce two such feedback mechanisms, based on cycles that appear in the mapping graph as well as on results returned by different peers.
