Approximating Document Frequency for Self-Index based Top-k Document Retrieval

Top-k document retrieval, which returns highly relevant documents relative to a query, is an essential task for many applications. One of the promising index frameworks is built by FM-index and wavelet tree for supporting efficient top-k document retrieval. The index, however, has difficulty on handling document frequency (DF) at search time because indexed terms are all substrings of a document collection. Previous works exhaustively search all the parts of the index, where most of the documents are not relevant, for DF calculation or store precalculated DF values in huge additional space. In this paper, we propose two methods to approximate DF of a query term by exploiting the information obtained from the process of traversing the index structures. Experimental results showed that our methods achieved almost equal effectiveness of exhaustive search while keeping search efficiency that time of our methods are about a half of the exhaustive search.