7969252
The role of theory today
The work of Hartmanis and Stearns, for which they rightly and belatedly won the Turing award, was both amazingly insightful and widely useful. It led to a long sequence of important innovation.s, such as work on cryptographic complexity, and it is a principal justification for theoretical research today. Further, theory has had triumphs outside the computational complexity area. Without wishing to be exhaustive, I could cite work on programming-language semantics that led to real advances in language design, the use of automaton-theoretic ideas in practical protocol verifiers, and many specific algorithms that have been used in practice. Yet looking back at the 1960s, I see a naive optimism that was based on too little evidence. The work on O(nlogn) sorting algorithms or Strassen’s matrix-multiplication algorithm, for example, caused us to believe that all problems would soon yield to algorithms that wene significantly more efficient than the obvious ones. LR(k ) parsing—a wonderful idea deeply rooted in language and automata theory—suggested that all aspects of compiling and other important systems software would soon yield to formalization and theoretical analysis. The theory community was not alone in its naivete. AI was similarly taken in by early successes, such as Samuels’s checker-playing program, to believe that very hard problems were in fact easy. This view has led to periodic, overly optimistic predictions of success that continue to trouble AI. Thus, despite the successes mentioned above, as the theory field grew there was a tendency to attack problems obliquely, assuming that we were contributing to an important goal. Papers would be written to address the issues raised by other papers. Then another round of work would address these papers, and so on. In the process, the real problems that spawned the original paper were forgotten, and the community began to imagine that the research embodied in the papers was the ultimate goal. (I confess to being as guilty of this behavior as anyone in the field.) Is the theory community therefore doomed to become ineffective within the broader discipline of computer science? I do not believe so for a moment. However, we need to reinvent theory for the next generation if we are to have the same impact now as a generation ago. Not everything needs to change. First, there is a core of theory, roughly the complexity theory outlined in the Hartmanis article, that needs to be pre-
291860
The TSIMMIS Approach to Mediation: Data Models and Languages
TSIMMIS—The Stanford-IBM Manager of Multiple InformationSources—is a system for integrating information. It offers a datamodel and a common query language that are designed to support thecombining of information from many different sources. It also offerstools for generating automatically the components that are needed tobuild systems for integrating information. In this paper we shalldiscuss the principal architectural features and their rationale.
5323612
A Query Translation Scheme for Rapid Implementation of Wrappers
Wrappers provide access to heterogeneous information sources by converting application queries into source specific queries or commands. In this paper we present a wrapper implementation toolkit that facilitates rapid development of wrappers. We focus on the query translation component of the toolkit, called the converter. The converter takes as input a Query Description and Translation Language (QDTL) description of the queries that can be processed by the underlying source. Based on this the converter decides if an application query is (a) directly supported, i.e., it can be translated to a query of the underlying system following instructions in the QDTL description; (b) logically supported, i.e., logically equivalent to a directly supported query; (c) indirectly supported, i.e., it can be computed by applying a filter, automatically generated by the converter, to the result of a directly supported query.
9277235
Index selection for OLAP
On-line analytical processing (OLAP) is a recent and important application of database systems. Typically, OLAP data is presented as a multidimensional "data cube." OLAP queries are complex and can take many hours or even days to run, if executed directly on the raw data. The most common method of reducing execution time is to precompute some of the queries into summary tables (subcubes of the data cube) and then to build indexes on these summary tables. In most commercial OLAP systems today, the summary tables that are to be precomputed are picked first, followed by the selection of the appropriate indexes on them. A trial-and-error approach is used to divide the space available between the summary tables and the indexes. This two-step process can perform very poorly. Since both summary tables and indexes consume the same resource-space-their selection should be done together for the most efficient use of space. The authors give algorithms that automate the selection of summary tables and indexes. In particular, they present a family of algorithms of increasing time complexities, and prove strong performance bounds for them. The algorithms with higher complexities have better performance bounds. However, the increase in the performance bound is diminishing, and they show that an algorithm of moderate complexity can perform fairly close to the optimal.
208920048
Integrating Information by Outerjoins and Full Disjunctions.
Our motivation is the piecing together of tidbits of information found on the "web" into a usable information structure. The problem is related to that of computing the natural outerjoin of many relations in a way that preserves all possible connections among facts. Such a computation has been termed a "full disjunction" by Galindo-Legaria. We are thus led to ask the question of when a full disjunction can be computed by some sequence of natural outerjoins. The answer involves a concept of from Fagin [1983] called fl-acyclic hypergraphs." We prove that there is a natural outerjoin sequence producing the full disjunction if and only if the set of relation schemes forms a connected, fl-acyclic hypergraph.
15385590
Dynamic itemset counting and implication rules for market basket data
We consider the problem of analyzing market-basket data and present several important contributions. First, we present a new algorithm for finding large itemsets which uses fewer passes over the data than classic algorithms, and yet uses fewer candidate itemsets than methods based on sampling. We investigate the idea of item reordering, which can improve the low-level efficiency of the algorithm. Second, we present a new way of generating “implication rules,” which are normalized based on both the antecedent and the consequent and are truly implications (not simply a measure of co-occurrence), and we show how they produce more intuitive results than other methods. Finally, we show how different characteristics of real data, as opposed by synthetic data, can dramatically affect the performance of the system and the form of the results.
14178054
The Transitive Reduction of a Directed Graph
We consider economical representations for the path information in a directed graph. A directed graph $G^t $ is said to be a transitive reduction of the directed graph G provided that (i) $G^t $ has a directed path from vertex u to vertex v if and only if G has a directed path from vertex u to vertex v, and (ii) there is no graph with fewer arcs than $G^t $ satisfying condition (i). Though directed graphs with cycles may have more than one such representation, we select a natural canonical representative as the transitive reduction for such graphs. It is shown that the time complexity of the best algorithm for finding the transitive reduction of a graph is the same as the time to compute the transitive closure of a graph or to perform Boolean matrix multiplication.
209399085
Answering Queries Using Limited External Processors.