| Similarity search in high-dimensional spaces is popular for applications like image processing, time series, and genome data. In higher dimensions, the phenomenon of curse of dimensionality kills the effectiveness of most of the index structures, giving way to approximate methods like Locality Sensitive Hashing (LSH), to answer similarity searches. In addition to range searches and k-nearest neighbor searches, there is a need to answer negative queries formed by excluded regions, in high-dimensional data. Though there have been a slew of variants of LSH to improve efficiency, reduce storage, and provide better accuracies, none of the techniques are capable of answering queries in the presence of excluded regions. This thesis provides a novel approach to handle such negative queries. This is achieved by creating a prefix based hierarchical index structure. First, the higher dimensional space is projected to a lower dimension space. Then, a one-dimensional ordering is developed, while retaining the hierarchical traits. The algorithm intelligently prunes the irrelevant candidates while answering queries in the presence of excluded regions. While naive LSH would need to filter out the negative query results from the main results, the new algorithm minimizes the need to fetch the redundant results in the first place. Experiment results show that this reduces post-processing cost thereby reducing the query processing time. |
