Optimization of real conjunctive queries

The optimization problem for conjunctive queries has been studied extensively. Unfortunately, this research almost invariably assumes set-theoretic semantics (i.e., duplicates are eliminated). In contrast, SQL queries have bag-theoretic semantics (i.e., in general duplicates are not eliminated). In this paper we study the optimization problems for conjunct ive queries under bagtheoretic semantics. We show that optimization techniques from the set-theoretic setting do not carry over to the bag-theoretic setting.

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