GSoC Developer Rebuilds Nominatim's Category Model to Support Multi-Tag Places

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Developer Agasta, selected for Google Summer of Code (GSoC), spent the summer overhauling the category model of Nominatim, the geocoder that converts OpenStreetMap place names and addresses into coordinates. The core problem was that Nominatim previously allowed only one class/type pair per place, making it difficult to represent objects with multiple identities, such as a hotel containing a restaurant. The rebuild introduced PostgreSQL's ltree extension to store categories as hierarchical dot-separated paths, enabling filters like 'all descendants of osm.amenity' without requiring explicit prefix storage. Changes spanned the import pipeline, database schema, ranking logic, search indexes, API, and SQLite adaptor, and also uncovered a pre-existing bug in the PostgreSQL array result processor. The updated API now supports advanced filtering with AND/OR semantics, hierarchy matching, and consistent behaviour across all supported backends.

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