Evolvable view environment (EVE): non-equivalent view maintenance under schema changes
SIGMOD '99 Proceedings of the 1999 ACM SIGMOD international conference on Management of data
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The problem of rewriting queries has been heavily explored in recent years, including in work on query processing and optimization, semantic query refinement in decentralized environments, the rewriting of queries using views, and view maintenance. Previous work has made the restricting assumption that the rewritten query must be {\em equivalent} to the initially given query. We now propose to relax this assumption to allow for query rewriting in situations where {\em equivalent} rewritings may not exist -- yet alternate {\em not necessarily equivalent} query rewritings may still be preferable to users over not receiving any answers at all. In this paper, we apply this concept of non-equivalent query rewriting to the problem of maintaining view definitions (data warehouses), where it now allows us to handle a much larger class of changes of the underlying information sources, namely not only data but also schema changes. This relaxed query notion allows for more diversity in acceptable query answers, but also raises a new issue of the evaluation of such queries. For this purpose, we introduce an analytical model of query rewritings that incorporates measures of information preservation (quality) of a query in addition to the commonly studied measures of view maintenance performance (query cost). Quality is modeled as a function of the divergence from the intended view extent, both in terms of the preservation of the information amount and the information type. Both quality and cost are integrated into one uniform model, called the Quality-Cost-Model or QC-Model, to allow for a trade-off among these two measures. This model can be used to compare two alternate (even if not equivalent) rewritings, and thus to automatically select a good view maintenance solution from among numerous non-equivalent rewritings. To our best knowledge, this is the first work that deals with this novel issue, the {\it non-equivalent query rewriting problem}.