Preferred Skyline: A Hybrid Approach Between SQLf and Skyline

The World Wide Web (WWW) is a great repository of data and it may reference thousands of data sources for almost any knowledge domain. Users frequently access sources to query information and may be interested only in the top k answers that meet their preferences. Although, many declarative languages have been defined to express WWW queries, the problem of specifying user preferences and considering this information during query optimization and evaluation remains open. Most used query languages, such as SQL and XQUERY, do not allow specifying general conditions on user preferences. For example, using the SQL ORDER BY clause one can express the order in which the answer will appear, but the user must be aware of filtering the tuples that satisfy their preferences. Skyline and SQLf are two extensions of SQL defined to express general user preferences. Skyline offers physical operators to construct a Pareto Curve of the non-dominated answers. Skyline may return answers with bad values for some criteria and does not discriminate between the non-dominated answers. On the other hand, SQLf gives the best answers in terms of user preferences, but it may return dominated answers. Finally, the skyline operator evaluation time is higher than SQLf. We proposed a hybrid approach, Preferred Skyline, integrating skyline and SQLf to produce only answers in the Pareto Curve with best satisfaction degrees. We report initial experimental results on execution time and answer precision. They show that Preferred Skyline consumes less time than Skyline and its precision is higher than SQLf.