Online SPARQL Aggregate Queries Processing with Web Preemption

Getting complete results when processing aggregate queries on public SPARQL endpoints is challenging, mainly due to quotas enforcement. Although the Web preemption allows to process aggregation queries online, on preemptable SPARQL servers, data transfer is still very large when processing count-distinct aggregate queries. In this paper, it is shown that count-distinct aggregate queries can be approximated with low data transfer by extending the partial aggregation operator with HyperLogLog sketches. Experimental results demonstrate that the proposed approach outperforms existing approaches by orders of magnitude in terms of the amount of transferred data.