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.
3104453
Implementing data cubes efficiently
Decision support applications involve complex queries on very large databases. Since response times should be small, query optimization is critical. Users typically view the data as multidimensional data cubes. Each cell of the data cube is a view consisting of an aggregation of interest, like total sales. The values of many of these cells are dependent on the values of other cells in the data cube. A common and powerful query optimization technique is to materialize some or all of these cells rather than compute them from raw data each time. Commercial systems differ mainly in their approach to materializing the data cube. In this paper, we investigate the issue of which cells (views) to materialize when it is too expensive to materialize all views. A lattice framework is used to express dependencies among views. We present greedy algorithms that work off this lattice and determine a good set of views to materialize. The greedy algorithm performs within a small constant factor of optimal under a variety of models. We then consider the most common case of the hypercube lattice and examine the choice of materialized views for hypercubes in detail, giving some good tradeoffs between the space used and the average time to answer a query.
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.
2826488
Computing Iceberg Queries Efficiently
Many applications compute aggregate functions (such as COUNT, SUM) over an attribute (or set of attributes) to find aggregate values above some specified threshold. We call such queries iceberg queries because the number of above-threshold results is often very small (the tip of an iceberg), relative to the large amount of input data (the iceberg). Such iceberg queries are common in many applications, including data warehousing, information-retrieval, market basket analysis in data mining, clustering and copy detection. We propose efficient algorithms to evaluate iceberg queries using very little memory and significantly fewer passes over data, as compared to current techniques that use sorting or hashing. We present an experimental case study using over three gigabytes of Web data to illustrate the savings obtained by our algorithms.
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.
6212306
Constraint checking with partial information
Constraints are a valuable tool for managing information across multiple databases, as well as for general purposes of assuring data integrity. However, efficient implementation of constraint checking is difficult. In this paper we explore techniques for assuring constraint satisfaction without performing a complete evaluation of the constraints. We consider methods that use only constraint definitions, methods that use constraints and updates, and methods that use constraints, updates, and “local” data.
17355782
On complementary and independent mappings on databases
We define the notion of independent views to indicate whether the range values of the two views may be achieved independently The concept of complementary views indicates when the domain element can be uniquely determined by the range values of the two complementary views We consider the relationship between independent and complementary views In unrestricted domains, a view (but not the identity or empty view) can have more than one complementary, independent view Databases, however, are more restricted domains They are finite power sets A view is monotonic if it preserves inclusion However, in finite power sets when all views are monotonic, if a given view has another view which is independent and complementary, then this view is unique.
209399085
Answering Queries Using Limited External Processors.
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.