Metric index: an efficient and scalable solution for precise and approximate similarity search

This publication doesn't include Faculty of Arts. It includes Faculty of Informatics. Official publication website can be found on



Type Article in Periodical
Magazine / Source Information Systems
MU Faculty or unit

Faculty of Informatics

Field Informatics
Keywords Metric space; Similarity search; Data structure; Approximation; Scalability
Description Metric space is a universal and versatile model of similarity that can be applied in various areas of information retrieval. However, a general, efficient, and scalable solution for metric data management is still a resisting research challenge. We introduce a novel indexing and searching mechanism called Metric Index (M-Index) that employs practically all known principles of metric space partitioning, pruning, and filtering, thus reaching high search performance while having constant building costs per object. The heart of the M-Index is a general mapping mechanism that enables to actually store the data in established structures such as the B+ - tree or even in a distributed storage. We implemented the M-Index with the B+ - tree and performed experiments on two datasets - the first is an artificial set of vectors and the other is a real-life dataset composed of a combination of five MPEG-7 visual descriptors extracted from a database of up to several million digital images. The experiments put several M-Index variants under test and compare them with established techniques for both precise and approximate similarity search. The trials show that the M-Index outperforms the others in terms of efficiency of search-space pruning, I/O costs, and response times for precise similarity queries. Further, the M-Index demonstrates excellent ability to keep similar data close in the index which makes its approximation algorithm very efficient - maintaining practically constant response times while preserving a very high recall as the dataset grows and even beating approaches designed purely for approximate search.
Related projects: