Answering Why-Not Group Spatial Keyword Queries

With the proliferation of geo-textual objects on the web, extensive efforts have been devoted to improving the efficiency of top-<inline-formula><tex-math notation="LaTeX">$k$</tex-math><alternatives><mml:math><mml:mi>k</mml:mi></mml:math><inline-graphic xlink:href="zheng-ieq1-2879819.gif"/></alternatives></inline-formula> spatial keyword queries in different settings. However, comparatively much less work has been reported on enhancing the quality and usability of such queries. In this context, we propose means of enhancing the usability of a top-<inline-formula><tex-math notation="LaTeX">$k$</tex-math><alternatives><mml:math><mml:mi>k</mml:mi></mml:math><inline-graphic xlink:href="zheng-ieq2-2879819.gif"/></alternatives></inline-formula> group spatial keyword query, where a group of users aim to find <inline-formula><tex-math notation="LaTeX">$k$</tex-math><alternatives><mml:math><mml:mi>k</mml:mi></mml:math><inline-graphic xlink:href="zheng-ieq3-2879819.gif"/></alternatives></inline-formula> objects that contain given query keywords and are nearest to the users. Specifically, when users receive the result of such a query, they may find that one or more objects that they expect to be in the result are in fact missing, and they may wonder why. To address this situation, we develop a so-called <italic>why-not</italic> query that is able to minimally modify the original query into a query that returns the expected, but missing, objects, in addition to other objects. Specifically, we formalize the <italic>why-not</italic> query in relation to the top-<inline-formula><tex-math notation="LaTeX">$k$</tex-math><alternatives><mml:math><mml:mi>k</mml:mi></mml:math><inline-graphic xlink:href="zheng-ieq4-2879819.gif"/></alternatives></inline-formula> group spatial keyword query, called the <underline>W</underline>hy-not <underline>G</underline>roup <underline>S</underline>patial <underline>K</underline>eyword Query (<inline-formula><tex-math notation="LaTeX">$\mathsf{WGSK}$</tex-math><alternatives><mml:math><mml:mi mathvariant="sans-serif">WGSK</mml:mi></mml:math><inline-graphic xlink:href="zheng-ieq5-2879819.gif"/></alternatives></inline-formula>) that is able to provide a group of users with a more satisfactory query result. We propose a three-phase framework for efficiently computing the <inline-formula><tex-math notation="LaTeX">$\mathsf{WGSK}$</tex-math><alternatives><mml:math><mml:mi mathvariant="sans-serif">WGSK</mml:mi></mml:math><inline-graphic xlink:href="zheng-ieq6-2879819.gif"/></alternatives></inline-formula>. The first phase substantially reduces the search space for the subsequent phases by retrieving a set of objects that may affect the ranking of the user-expected objects. The second phase provides an incremental sampling algorithm that generates candidate weightings of more promising queries. The third phase determines the penalty of each refined query and returns the query with minimal penalty, i.e., the minimally modified query. Extensive experiments with real and synthetic data offer evidence that the proposed solution excels over baselines with respect to both effectiveness and efficiency.