On the Impact of Random Index-Partitioning on Index Compression

Abstract: The performance of processing search queries depends heavily on the stored index size. Accordingly, considerable research efforts have been devoted to the development of efficient compression techniques for inverted indexes. Roughly, index compression relies on two factors: the ordering of the indexed documents, which strives to position similar documents in proximity, and the encoding of the inverted lists that result from the ordered stream of documents. Large commercial search engines index tens of billions of pages of the ever growing Web. The sheer size of their indexes dictates the distribution of documents among thousands of servers in a scheme called local index-partitioning, such that each server indexes only several millions pages. Due to engineering and runtime performance considerations, random distribution of documents to servers is common. However, random index-partitioning among many servers adversely impacts the resulting index sizes, as it decreases the effectiveness of document ordering schemes. We study the impact of random index-partitioning on document ordering schemes. We show that index-partitioning decreases the aggregated size of the inverted lists logarithmically with the number of servers, when documents within each server are randomly reordered. On the other hand, the aggregated partitioned index size increases logarithmically with the number of servers, when state-of-the-art document ordering schemes, such as lexical URL sorting and clustering with TSP, are applied. Finally, we justify the common practice of randomly distributing documents to servers, as we qualitatively show that despite its ill-effects on the ensuing compression, it decreases key factors in distributed query evaluation time by an order of magnitude as compared with partitioning techniques that compress better.