Making queries tractable on big data with preprocessing

A query class is traditionally considered tractable if there exists a polynomial-time (PTIME) algorithm to answer its queries. When it comes to big data, however, PTIME algorithms often become infeasible in practice. A traditional and effective approach to coping with this is to preprocess data off-line, so that queries in the class can be subsequently evaluated on the data efficiently. This paper aims to provide a formal foundation for this approach in terms of computational complexity. (1) We propose a set of Π-tractable queries, denoted by ΠT Q 0 , to characterize classes of queries that can be answered in parallel poly-logarithmic time (NC) after PTIME preprocessing. (2) We show that several natural query classes are Π-tractable and are feasible on big data. (3) We also study a set ΠT Q of query classes that can be effectively converted to Π-tractable queries by refactorizing its data and queries for preprocessing. We introduce a form of NC reductions to characterize such conversions. (4) We show that a natural query class is complete for ΠT Q . (5) We also show that ΠT Q 0 ⊂ P unless P = NC, i.e., the set ΠT Q 0 of all Π-tractable queries is properly contained in the set P of all PTIME queries. Nonetheless, ΠT Q = P, i.e., all PTIME query classes can be made Π-tractable via proper refactorizations. This work is a step towards understanding the tractability of queries in the context of big data.