Online Ordering of Overlapping Data Sources

Data integration systems offer a uniform interface for querying a large number of autonomous and heterogeneous data sources. Ideally, answers are returned as sources are queried and the answer list is updated as more answers arrive. Choosing a good ordering in which the sources are queried is critical for increasing the rate at which answers are returned. However, this problem is challenging since we often do not have complete or precise statistics of the sources, such as their coverage and overlap. It is further exacerbated in the Big Data era, which is witnessing two trends in Deep-Web data: first, obtaining a full coverage of data in a particular domain often requires extracting data from thousands of sources; second, there is often a big variation in overlap between different data sources. In this paper we present OASIS, an Online query Answering System for overlappIng Sources. OASIS has three key components for source ordering. First, the Overlap Estimation component estimates overlaps between sources according to available statistics under the Maximum Entropy principle. Second, the Source Ordering component orders the sources according to the new contribution they are expected to provide, and adjusts the ordering based on statistics collected during query answering. Third, the Statistics Enrichment component selects critical missing statistics to enrich at runtime. Experimental results on both real and synthetic data show high efficiency and scalability of our algorithm.