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Recent Submissions

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    State-complexity hierarchies of uniform languages of alphabet-size length
    (University of Waterloo, 2008-03-11) Brzozowski, Janusz; Konstantinidis, Stavros
    We study the state complexity of a special class of simple languages. If A is an alphabet of k letters, then a k-language is a nonempty set of words of length k, that is, a uniform language of length k. We show that every k-language of maximal state complexity is also a uniform language of length k of maximal state complexity. Moreover, we prove that, for every i between the minimal and the maximal state complexities, there is a language of complexity i. The proof is constructive: for each i we exhibit a language of complexity i. We introduce a family of "pi automata" accepting languages whose words are permutations of the alphabet; the complexities of these languages form a complete hierarchy between k^2-k+3 and 2^k+1. We start with an automaton with k^2-k+3 states and show that states can be added one at a time, until the automaton has 2^k+1 states. We construct another family of automata, based on k-ary trees, whose languages define a complete hierarchy of complexities between 2^k+1 and the maximal complexity. Here, we start with an automaton with the maximal complexity. Here, we start with an automaton with the maximal number of states and remove states one at a time, until an automaton with 2^k+1 and the maximal complexity. Here, we start with an automaton with the maximal number of states and remove states one at a time, until an automaton with 2^k+1 states is reached.
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    Extending typestate analysis to multiple interacting objects
    (Univeristy of Waterloo, 2008-03-25) Naeem, Nomair A.; Lhotak, Ondrej
    This paper extends static typestate analysis to temporal specifications of groups of interacting objects, which are expressed using tracematches. Unlike typestate, a tracematch state may change due to operations on any of a set of objects bound by the tracematch. The paper proposes a lattice-based operational semantics equivalent to the original tracematch semantics but better suited to statis analysis. The paper defines a statis analysis that computes precise local points-to sets and tracks the flow of individual objects, thereby enabling strong state updates of the tracematch state. The analysis has been proved sound with respect to the semantics. A context-sensitive version of the analysis has been implemented as instances of the IFDS and IDE algorithms. The analysis was evaluated on tracematches used in earlier work and found to be very precise. Remaining imprecisions could be eliminated with more precise modeling of references from the heap and of exceptional control flow.
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    JTop algorithms for top-k join queries
    (University of Waterloo, 2008-03-25) Akbarinia, Reza; Ilyas, Ihab F.; Ozsu, M. Tamer; Valduriez, Patrick
    Top-k join queries are very important, because there are many applications in which users need to join multiple inputs and are interested i the top-k join results based on some scoring function that combines some attribute values of each input. One of the most efficient algorithms for top-k join queries is the Rank-Join algorithm. In this report, we first study this algorithm and show that there are many cases where its threshold is lazy, i.e. decreases very slowly, and the algorithm needs to go too far in the lists. Then, we propose a family of efficient algorithms for processing top-k join queries. The main idea is to take advantage of the specific information on join attribute values as well as characteristics of the query and of the underlying system. Our contributions are as follows. First, we propose a general model for the problem of top-k join queries which is useful for databases as well as many other areas of computing. Second, we propose JTop, an efficient top-k join algorithm for systems where random accesses are not expensive. JTop takes advantage of both random and sorted accesses as well as information on the join condition. In contrast to Rank-Join, JTop's threshold is not lazy if at least one of the scoring attributes has a reasonable progressive impact on the scoring function. Third, we propose two new algorithms, LR_JTop and NR_JTop, for systems where random accesses are expensive or not supported, respectively. Forth, we propose a new algorithm called BP_JTop which is designed for systems with position-based indexing, i.e. when accessing a data item, the index gives its position. For each of our algorithms, we prove that over any database, it stops before or at the same position at which Rank-Join stops. We also show that there is a class of databases over which our algorithm stops at a position that is O(n) times lower than that of Rank-Join, where n is the number of data items. We also conducted an extensive experimental study to evaluate the performance of our algorithms under different data distributions. The performance evaluation shows that our algorithms obtain high performance gains against the Rank-Join algorithm.
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    Worst case optimal union-intersection expression evaluation
    (University of Waterloo, 2008-02-08) Chiniforooshan, Ehsan; Farzan, Arash; Mirzazadeh
    We consider the problem of evaluating an expression consisting of unions and intersections of some sorted sets in the comparison model. Given the expression and the sizes of the sets, we are interested in the worst-case complexity of evaluating the expression in terms of the sizes of the sets. We assume the sets in the given expression are independent. We show a lower bound on this problem and present an algorithm that matches the lower bound asymptotically.
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    A performance evaluation evaluation of database systems on virtual machines
    (University of Waterloo, 2008-01-23) Minhas, Umar Farooq
    Virtual machine technologies offer simple and practical mechanisms to address many manageability problems in database systems. For example, these technologies allow for server consolidation, easier deployment, and more flexible provisioning. Therefore, database systems are increasingly being run on virtual machines. This offers many unique opportunities for database research. However, it is also important to understand the cost of virtualization. Virtual machine technologies add a layer of indirection between applications and the hardware that they use (e.g. CPU, memory, disk). This added complexity results in a performance overhead for software systems running in a virtual machine. In this thesis, we present an experimental study of the overhead of running a database workload in a virtual machine. Using a TPC-H workload running on PostgreSQL in a Xen virtual machine environment, we show that Xen does indeed introduce overhead for system calls, page fault handling, and disk I/O. However, these overheads do not translate to a high overhead in query execution time. We show that in all cases the average overhead is less than 10% and, therefore, conclude that the advantages of running a database system in a virtual machine do not come at a high cost in performance.