Abstract-Data sources on the web are often accessible through web interfaces that present them as relational tables, but require certain attributes to be mandatorily selected, e.g., via a web form. In a scenario where we integrate a set of such sources, and we pose queries over them, the values needed to access a source may have to be retrieved from other sources that are possibly not even mentioned in the query: answering queries at best can then be done only with a potentially recursive query plan that gets all obtainable answers to the query. Since data sources are typically distributed over a network, a major cost indicator for the execution of a query plan is the number of accesses to remote sources. In this paper we present an optimization technique for conjunctive queries that produces a query plan that: (1) minimizes the number of accesses according to a strong notion of minimality; (2) excludes all sources that are not relevant for the query. We introduce Toorjah, a prototype system that answers queries posed on sources with limitations by means of optimized query plans. Toorjah adopts a strategy that is aimed to retrieve answers as early as possible during query processing, and to present them to the user as they are computed. We provide experimental evidence of the effectiveness of our optimization, by showing the reduction of the number of accesses in a large number of cases. I. INTRODUCTIONIn the context of integration of web data [1], or in a data exchange setting where source data are retrieved on the web, information is often accessible only via forms; it is easy to see that accessing data through a web form amounts to querying a relational table, where a selection is specified by the fields that are filled in. Typically, certain fields are required to be filled in by the user in order to obtain a result; for example, all online shops forbid a request posed by a user who leaves all fields of the search form empty. Analogously, in legacy systems where data are scattered over several files and wrapped as relational tables, similar limitations are enforced.Limitations on how sources can be accessed significantly complicate query processing: as shown, e.g., in [2], [3], query answering in the presence of access limitations in general requires the evaluation of a recursive query plan. This is shown in the following example.Example 1: Suppose we have three relations: r 1 (Artist, Nation, YOB ), which stores artists with their nationality and year of birth, and requires the first attribute to be selected; r 2 (Title, Year , Artist), which stores data
The following literature review covers journal articles related to biosolids and sludge management that were published in 2000. The main topics covered include regulations and planning, reduction of biosolids and sludge production, biosolids and sludge characteristics, thickening, conditioning and dewatering, thermal processes, land application, alternative uses for biosolids, disposal, and articles relating to overall biosolids programs. These topics have been generated by increased regulations and scrutiny of biosolids programs, and the need for continued research and education in the field of biosolids management. REGULATIONS AND PLANNINGConcerns over possible health effects resulting from sludge land application use have reportedly initiated discussions between the EPA and the National Academy of Sciences. Rennier (2000) reported that the EPA may be interested in re-evaluating the 503 regulations. This re-evaluation would focus on human health issues, risk assessment, and ecological issues. Canning (2000) discussed the EPA's proposed dioxin standard of 300 ppt of toxic equivalent for biosolids used in land application.This will effect many treatment plants that exceed these limits indicating that they will be required to find alternative methods of disposal. Conrad (2000) stated that Section 503 regulations are not working, citing examples of improper use of sewage sludge and numerous non-protective aspects of the current standards. Three recommendations were proposed for enhancing Section 503 safeguards: 1) enforce more rigorous monitoring requirements; 2) require limits on infrequently occurring metals and biological contaminants; and 3) increase public awareness and notice of treated land, and universal labeling of sewage sludge products. Birchfield and Stetar (2000) indicated that traditional biosolids handling and disposal methods such as dewatering or incineration can concentrate radionucleotides. The proper management of the land
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