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Register a new userParallel-Correctness and Transferability for Conjunctive Queries
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Thomas Schwentick
Publication date
2014
fields
Informatics Engineering
and research's language is
English
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A dominant cost for query evaluation in modern massively distributed systems is the number of communication rounds. For this reason, there is a growing interest in single-round multiway join algorithms where data is first reshuffled over many servers and then evaluated in a parallel but communication-free way. The reshuffling itself is specified as a distribution policy. We introduce a correctness condition, called parallel-correctness, for the evaluation of queries w.r.t. a distribution policy. We study the complexity of parallel-correctness for conjunctive queries as well as transferability of parallel-correctness between queries. We also investigate the complexity of transferability for certain families of distribution policies, including, for instance, the Hypercube distribution.
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Single-round multiway join algorithms first reshuffle data over many servers and then evaluate the query at hand in a parallel and communication-free way. A key question is whether a given distribution policy for the reshuffle is adequate for computing a given query, also referred to as parallel-correctness. This paper extends the study of the complexity of parallel-correctness and its constituents, parallel-soundness and parallel-completeness, to unions of conjunctive queries with and without negation. As a by-product it is shown that the containment problem for conjunctive queries with negation is coNEXPTIME-complete.
Structural indexing is an approach to accelerating query evaluation, whereby data objects are partitioned and indexed reflecting the precise expressive power of a given query language. Each partition block of the index holds exactly those objects that are indistinguishable with respect to queries expressible in the language. Structural indexes have proven successful for XML, RDF, and relational data management. In this paper we study structural indexing for conjunctive path queries (CPQ). CPQ forms the core of contemporary graph query languages such as SPARQL, Cypher, PGQL, and G-CORE. CPQ plays the same fundamental role with respect to contemporary graph query languages as the classic conjunctive queries play for SQL. We develop the first practical structural indexes for this important query language. In particular, we propose a structural index based on k-path-bisimulation, tightly coupled to the expressive power of CPQ, and develop algorithms for efficient query processing with our index. Furthermore, we study workload-aware structural indexes to reduce both the construction and space costs according to a given workload. We demonstrate through extensive experiments using real and synthetic graphs that our methods accelerate query processing by up to multiple orders of magnitude over the state-of-the-art methods, without increasing index size.
We consider the task of enumerating and counting answers to $k$-ary conjunctive queries against relational databases that may be updated by inserting or deleting tuples. We exhibit a new notion of q-hierarchical conjunctive queries and show that these can be maintained efficiently in the following sense. During a linear time preprocessing phase, we can build a data structure that enables constant delay enumeration of the query results; and when the database is updated, we can update the data structure and restart the enumeration phase within constant time. For the special case of self-join free conjunctive queries we obtain a dichotomy: if a query is not q-hierarchical, then query enumeration with sublinear$^ast$ delay and sublinear update time (and arbitrary preprocessing time) is impossible. For answering Boolean conjunctive queries and for the more general problem of counting the number of solutions of k-ary queries we obtain complete dichotomies: if the querys homomorphic core is q-hierarchical, then size of the the query result can be computed in linear time and maintained with constant update time. Otherwise, the size of the query result cannot be maintained with sublinear update time. All our lower bounds rely on the OMv-conjecture, a conjecture on the hardness of online matrix-vector multiplication that has recently emerged in the field of fine-grained complexity to characterise the hardness of dynamic problems. The lower bound for the counting problem additionally relies on the orthogonal vectors conjecture, which in turn is implied by the strong exponential time hypothesis. $^ast)$ By sublinear we mean $O(n^{1-varepsilon})$ for some $varepsilon>0$, where $n$ is the size of the active domain of the current database.
Structural decomposition methods have been developed for identifying tractable classes of instances of fundamental problems in databases, such as conjunctive queries and query containment, of the constraint satisfaction problem in artificial intelligence, or more generally of the homomorphism problem over relational structures. Most structural decomposition methods can be characterized through hypergraph games that are variations of the Robber and Cops graph game that characterizes the notion of treewidth. In particular, decomposition trees somehow correspond to monotone winning strategies, where the escape space of the robber on the hypergraph is shrunk monotonically by the cops. In fact, unlike the treewidth case, there are hypergraphs where monotonic strategies do not exist, while the robber can be captured by means of more complex non-monotonic strategies. However, these powerful strategies do not correspond in general to valid decompositions. The paper provides a general way to exploit the power of non-monotonic strategies, by allowing a disciplined form of non-monotonicity, characteristic of cops playing in a greedy way. It is shown that deciding the existence of a (non-monotone) greedy winning strategy (and compute one, if any) is tractable. Moreover, despite their non-monotonicity, such strategies always induce valid decomposition trees, which can be computed efficiently based on them. As a consequence, greedy strategies allow us to define new islands of tractability for the considered problems properly including all previously known classes of tractable instances.
In this paper, we focus on the problem of determining whether two conjunctive (CQ) queries posed on relational data are combined-semantics equivalent [9]. We continue the tradition of [2,5,9] of studying this problem using the tool of containment between queries. We introduce a syntactic necessary and sufficient condition for equivalence of queries belonging to a large natural language of explicit-wave combined-semantics CQ queries; this language encompasses (but is not limited to) all set, bag, and bag-set queries, and appears to cover all combined-semantics CQ queries that are expressible in SQL. Our result solves in the positive the decidability problem of determining combined-semantics equivalence for pairs of explicit-wave CQ queries. That is, for an arbitrary pair of combined-semantics CQ queries, it is decidable (i) to determine whether each of the queries is explicit wave, and (ii) to determine, in case both queries are explicit wave, whether or not they are combined-semantics equivalent, by using our syntactic criterion. (The problem of determining equivalence for general combined-semantics CQ queries remains open. Even so, our syntactic sufficient containment condition could still be used to determine that two general CQ queries are combined-semantics equivalent.) Our equivalence test, as well as our general sufficient condition for containment of combined-semantics CQ queries, reduce correctly to the special cases reported in [2,5] for set, bag, and bag-set semantics. Our containment and equivalence conditions also properly generalize the results of [9], provided that the latter are restricted to the language of (combined-semantics) CQ queries.
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