Honored for his dissertation on scalable spatial data management
Thanasis Georgiadis, postdoc in the DIMA research group led by BIFOLD Co-director Volker Markl, has received the Hellenic ACM SIGMOD Doctoral Dissertation Award 2026 in Athens. The award recognizes his dissertation "Scalable management of complex spatial data types", work that speeds up spatial join processing on complex geographic data types with a wide variety of applications from high-performance, distributed spatial data management systems to spatial reasoning tasks over natural language.
His dissertation addresses three closely related challenges in scalable spatial data management. First, the evaluation of spatial and topological joins, a fundamental operation for tasks such as geospatial interlinking, becomes computationally expensive when applied to polygons with a high number of vertices, as exact geometric intersection testing scales poorly with geometric complexity. To address this, Georgiadis developed low-memory-footprint approximation and filtering techniques that resolve joins without accessing the original, complex geometries wherever possible, invoking exact computation only as a last resort. Second, to ensure scalability under growing data volumes and geometric complexity, he designed and implemented a high-performance, distributed prototype framework that integrates state-of-the-art indexing, approximation, and filtering techniques while minimizing communication overhead and memory consumption. Third, the dissertation extends these contributions to the domain of large language models, which typically exhibit weak performance on spatial reasoning tasks absent costly fine-tuning. By leveraging the developed topological algorithms to compute spatial relations at scale and representing them as RDF triples, Georgiadis employed retrieval-augmented generation to supply this relational knowledge to language models at inference time, improving the factual accuracy of their responses to spatial queries.
Publication:
DOI: 10.12681/eadd/59951
The DIMA group had a strong presence at the Hellenic Data Management Symposium (HDMS) 2026: Alongside the award, Thanasis and his co-student Yannik Schröder gave a tutorial titled "A Tutorial on NebulaStream: Stream Processing in the Cloud-Edge Continuum," introducing participants to NebulaStream's architecture and showing how to extend it with custom operators, functions, and end-to-end tests, illustrated through a smart city traffic light scenario. BIFOLD alumni Eleni Tzirita Zacharatou (HPI) and Haralampos Gavriilidis (University of Carlifornia, Berkeley) also attended and presented their work on "Sheetreader."
