Efficient Query Processing for Tracing RFID Tags by Reordering Location Identifiers

This paper addresses the problem of using the location identifier (LID) as the domain value of the index for trajectories of RFID tags and proposes the solution for solving this problem. The query performance for tracing tags depends upon the distribution of tag trajectories in the data space. We investigate a more efficient representation of tag trajectories by means of ordering the set of values in a 3-dimensional domain. Our analysis shows that the order of LIDs makes a greater contribution to the efficiency of query processing, compared with other domain values. However, there is no rule of assigning an LID to the RFID location in order to process queries efficiently. To solve this problem, we propose a new LID proximity function to rearrange an arbitrary order of LIDs. This function enables logically adjacent tag trajectories, which are accessed simultaneously, to be stored in close proximity on the disk. To determine the optimal sequence of LIDs in the domain, we also propose a reordering scheme of LIDs. Our experiments show that the proposed reordering scheme improves the performance of queries, compared with the previous method of assigning LIDs.