Xiangxi Zheng, Kuang He, Jiayi Hu, Ping Yu, Rui Yan, Yuan Yao, Peng Hou, Anxiang Zeng, Alex Jinpeng Wang. Proceedings of the 64th Annual Meeting of the Association for Computational Linguistics (Volume 1: Long Papers). 2026.

Indexes in a database system can consume a large amount of memory. When they grow too large to be entirely held in the memory, selected portions of the indexes have to be unloaded to the secondary storage. There are a number of challenges in the design of an extensible index spanning memory and disk. First, the designs of in-memory portion and on-disk portion of the index must be decoupled so that the best choice for each device can be independently made. Second, selective unloading of in-memory portion to the disk must be carefully designed to maximize chance of memory access and to produce the most disk-friendly I/O access. Third, the strategy for index reloading from the disk and retaining in the memory must be optimized for the highest memory efficiency. In this paper, we proposed a memory-disk-spanning index design, named IndeXY, to effectively address the challenges. IndeXY distinguishes itself by being a framework that allows separate adoption of an in-memory index design and an on-disk data organization and access scheme that are deemed most efficient to its workloads. Instead of being just another one-size-fit-all index across memory and disk, the framework provides well-designed mechanisms and policies to integrate a selected in-memory index (Index X) and an on-disk index (Index Y) into one extensible index (IndeXY). We have implemented IndeXY with alternative in-memory indexes (ART tree or B+ tree) and alternative disk indexes (LSM tree or B+ tree). As an anecdotal example, experiments show that integrating the ART tree and an LSM tree in the framework can lead to a throughput improvement by as high as an 8.6X on a TPC-C workload over LeanStore that uses B+-tree indexes in the memory and disk, and can improve performance for almost all YCSB workloads.

Based on wave-interferometry principles, we describe a procedure to synthesize monostatic and multioffset borehole radar reflection data from cross-hole radar tomography data. The procedure is equivalent to placing multiple transmitting sources in the receiving hole and conducting wide-angle reflection and refraction surveys. The procedure is illustrated using transmission and reflection data generated by numerical simulation of electromagnetic waves for a simple fracture model. The numerically simulated reflection data compare favorably with the reflection data synthesized using the wave-interferometry method. An experimental test of this procedure is also applied to a piece of field data from a pair of boreholes in crystalline igneous bedrock. The results demonstrate the potential of practical use of wave-interferometry methods for extracting reflection information from cross-hole radar tomography data.