Abstract We introduce the R everse S patial Top-k K eyword (RSK) query, which is defined as: given a query term q, an integer k and a neighborhood size find all the neighborhoods of that size where q is in the top-k most frequent terms among the social posts in those neighborhoods . An obvious approach would be to partition the dataset with a uniform grid structure of a given cell size and identify the cells where this term is in the top-k most frequent keywords. However, this answer would be incomplete since it only checks for neighborhoods that are perfectly aligned with the grid. Furthermore, for every neighborhood (square) that is an answer, we can define infinitely more result neighborhoods by minimally shifting the square without including more posts in it. To address that, we need to identify contiguous regions where any point in the region can be the center of a neighborhood that satisfies the query. We propose an algorithm to efficiently answer an RSK query using an index structure consisting of a uniform grid augmented by materialized lists of term frequencies. We apply various optimizations that drastically improve query latency against baseline approaches. We also provide a theoretical model to choose the optimal cell size for the index to minimize query latency. We further examine a restricted version of the problem (RSKR) that limits the scope of the answer and propose efficient approximate algorithms. Finally, we examine how parallelism can improve performance by balancing the workload using a smart load slicing technique. Extensive experimental performance evaluation of the proposed methods using real Twitter datasets and crime report datasets, shows the efficiency of our optimizations and the accuracy of the proposed theoretical model.

The wide availability of tracking devices has drastically increased the role of geolocation in social networks, resulting in new commercial applications; for example, marketers can identify current trending topics within a region of interest and focus their products accordingly. In this paper we study a basic analytics query on geotagged data, namely: given a spatiotemporal region, find the most frequent terms among the social posts in that region. While there has been prior work on keyword search on spatial data (find the objects nearest to the query point that contain the query keywords), and on group keyword search on spatial data (retrieving groups of objects), our problem is different in that it returns keywords and aggregated frequencies as output, instead of having the keyword as input. Moreover, we differ from works addressing the streamed version of this query in that we operate on large, disk resident data and we provide exact answers. We propose an index structure and algorithms to efficiently answer such top-k spatiotemporal range queries, which we refer as Top-k Frequent Spatiotemporal Terms (kFST) queries. Our index structure employs an R-tree augmented by top-k sorted term lists (STLs), where a key challenge is to balance the size of the index to achieve faster execution and smaller space requirements. We theoretically study and experimentally validate the ideal length of the stored term lists, and perform detailed experiments to evaluate the performance of the proposed methods compared to baselines on real datasets.