Effect of different docid orderings on dynamic pruning retrieval strategies

Document-at-a-time (DAAT) dynamic pruning strategies for information retrieval systems such as MaxScore and Wand can increase querying efficiency without decreasing effectiveness. Both work on posting lists sorted by ascending document identifier (docid). The order in which docids are assigned - and hence the order of postings in the posting lists - is known to have a noticeable impact on posting list compression. However, the resulting impact on dynamic pruning strategies is not well understood. In this poster, we examine the impact on the efficiency of these strategies across different docid orderings, by experimenting using the TREC ClueWeb09 corpus. We find that while the number of postings scored by dynamic pruning strategies do not markedly vary for different docid orderings, the ordering still has a marked impact on mean query response time. Moreover, when docids are assigned by lexicographical URL ordering, the benefit to response time for is more pronounced for Wand than for MaxScore.