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Using views to generate efficient evaluation plans for queries (2007)
| Citations: | 4 - 3 self |
BibTeX
@MISC{Afrati07usingviews,
author = {Foto N. Afrati and Chen Li and Jeffrey D. Ullman},
title = {Using views to generate efficient evaluation plans for queries },
year = {2007}
}
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Abstract
We study the problem of generating efficient, equivalent rewritings using views to compute the answer to a query. We take the closed-world assumption, in which views are materialized from base relations, rather than views describing sources in terms of abstract predicates, as is common when the open-world assumption is used. In the closed-world model, there can be an infinite number of different rewritings that compute the same answer, yet have quite different performance. Query optimizers take a logical plan (a rewriting of the query) as an input, and generate efficient physical plans to compute the answer. Thus our goal is to generate a small subset of the possible logical plans without missing an optimal physical plan. We first consider a cost model that counts the number of subgoals in a physical plan, and show a search space that is guaranteed to include an optimal rewriting, if the query has a rewriting in terms of the views. We also develop an efficient algorithm for finding rewritings with the minimum number of subgoals. We then consider a cost model that counts the sizes of intermediate relations of a physical plan, without dropping any attributes, and give a search space for finding optimal rewritings. Our final cost model allows attributes to be dropped in intermediate relations. We show that, by careful variable renaming, it is possible to do better than the standard “supplementary relation ” approach, by dropping attributes that the latter approach would retain. Experiments show that our algorithm of generating optimal rewritings has good efficiency and scalability.
Keyphrases
efficient evaluation plan optimal rewriting intermediate relation search space cost model physical plan optimal physical plan minimum number equivalent rewriting standard supplementary relation approach possible logical plan infinite number closed-world model latter approach logical plan efficient algorithm closed-world assumption good efficiency different performance base relation open-world assumption query optimizers small subset abstract predicate careful variable renaming different rewriting efficient physical plan final cost model
