[Tile-serving] [openstreetmap/osm2pgsql] Ideas for improved expire mechanism (Issue #1662)

[Andy Allan]

The list above sounds good to me, and I'm particularly interested in the symmetrical difference based expiry for large polygon features (since OSM has a lot of giant ice sheets, protected areas, admin boundaries and other things that change shape often but without needing to refresh everything within the entire unioned extent). I could add "outline expiry" to the list of options, which is a bit like the symmetrical difference but only along the perimeters of the polygons (useful where they are only stroked and not filled, so no need to expire the zone between the old and new boundaries) but I suspect the real-world usefulness of this concept is limited, given the number of polygon changes that would move edges more than typical z14-ish expiry-tile sized distances.

The one other topic to consider is per-feature buffering for expiry. For example, if a place name changes, then not only the tile that contains the point needs expiring, but also other tiles within a certain radius from that point will need expiring too, since labels will generally span multiple expiry-tiles. Feature buffers are also needed for polygons and lines (either for boundaries/lines drawn with a thickness, or for labels along boundaries/lines), but place label points illustrate the need the most.

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