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This paper is concerned with a transformation-based approach to update propagation in an extended version of Codd's relational algebra which allows for defining derived relations (even recursively). It is shown that the desired optimization effects of update propagation may be lost if no generalized selection pushing strategy is employed to the transformed algebra expressions. A possible solution is the application of the Magic Sets rewriting but this may lead to unstratifiability of the incremental expressions. For the efficient evaluation of Magic Sets transformed algebra expressions they propose to use the soft stratification approach because of the simplicity and efficiency of this technique.
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