Where are you heading, metric access methods?: a provocative survey

In this paper the impact of the metric indexing paradigm on the real-world applications is discussed. We pose questions whether the priorities in research of metric access methods (MAMs) established in the past decades reflect the actual needs of practitioners. In particular, we formulate the following pragmatic questions: Are the established MAM cost measures relevant? Isn't the metric space model too general when the majority of real-world applications use Lp spaces? On the other hand, isn't the metric model too restrictive with respect to the growing community of practitioners using non-metric distances? Are the simple similarity queries competitive enough? Have the real-world similarity search engines ever used a general metric access method, or do they use specific indexing? Is there a real demand for content-based similarity search or will the annotations and keyword search win the game? We present justification of these questions, investigating relevant literature and search engines. Finally, we try to transform the questions into answers and suggestions to the future research on MAMs.