Upper Bound Approximations for BlockMaxWand

BlockMaxWand is a recent advance on theWand dynamic pruning technique, which allows ecient retrieval without any e‚ectiveness degradation to rank K . However, while BMW uses docid-sorted indices, it relies on recording the upper bound of the term weighting model scores for each block of postings in the inverted index. Such a requirement can be disadvantageous in situations such as when an index must be updated. In this work, we examine the appropriateness of upper-bound approximation – which have previously been shown suitable for Wand– in providing ecient retrieval for BMW. Experiments on the ClueWeb12 category B13 corpus using 5000 queries from a real search engine’s query log demonstrate that BMW still provides bene€ts w.r.t. Wand when approximate upper bounds are used, and that, if approximations on upper bounds are tight, BMW with approximate upper bounds can provide eciency gains w.r.t.Wandwith exact upper bounds, in particular for queries of short to medium length.