[Tile-serving] [osm2pgsql-dev/osm2pgsql] Difference-based expire (Issue #2454)

[Paul Norman]

pnorman left a comment (osm2pgsql-dev/osm2pgsql#2454)

I'm thinking about this from the perspective of water, where you have a geometry for the area and another for a label point if named. A member node movement might result in no changes for the point and only a change in part of the polygon.

Should we look at columns? If a member node change can avoid expiring because the geometry and tags are the same that means a tag change that doesn't result in any DB column changes is also safe to not expire by the same logic.

I'm also not sure how this affects two-stage processing. In the water example you'd never use two-stage processing. I've mainly used two-stage for forming border linestrings. In a raster tile scenario those would never be safe to do a difference-based expire since the distance along the linestring matters so a change at the start can change the entire line rendering. In a vector tile scenario it would be safe.



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