Abstract. A classical problem in query optimization is to find the optimal ordering of a set of possibly correlated selections. We provide an abstraction of this problem as a generalization of set cover called pipelined set cover, where the sets are applied sequentially to the elements to be covered and the elements covered at each stage are discarded. We show that several natural heuristics for this NP-hard problem, such as the greedy set-cover heuristic and a local-search heuristic, can be analyzed using a linear-programming framework. These heuristics lead to efficient algorithms for pipelined set cover that can be applied to order possibly correlated selections in conventional database systems as well as datastream processing systems. We use our linear-programming framework to show that the greedy and local-search algorithms are 4-approximations for pipelined set cover. We extend our analysis to minimize the lp-norm of the costs paid by the sets, where p ≥ 2 is an integer, to examine the improvement in performance when the total cost has increasing contribution from initial sets in the pipeline. Finally, we consider the online version of pipelined set cover and present a competitive algorithm with a logarithmic performance guarantee. Our analysis framework may be applicable to other problems in query optimization where it is important to account for correlations. A common operation in database query processing is to find the subset of records in a relation that satisfy a given set of selection conditions. To execute this operation efficiently, a query processor prefers to determine the optimal order in which to evaluate the individual selection conditions, so we call this operation pipelined filters [2, 4, 12, 18] . Optimality in pipelined filters is usually with respect to minimizing the total processing time [4, 12] . For example, consider a relation packets, where each record contains the header and an initial part of the payload of network packets logged by a network
There has been previous work on online algorithm for pipelined set-cover problem REF , where the sets are coming online.
The pipelined set cover problem
Computer Science