[Tile-serving] [osm2pgsql-dev/osm2pgsql] Add grouped-linemerge generalization (PR #2482)

[Sarah Hoffmann]

lonvia left a comment (osm2pgsql-dev/osm2pgsql#2482)

The tricky part with updates here is that your algorithm runs while the tables are in a in-between state: the source table has already all the updated tags and geometries. Deleted ways are gone from the source table without a trace. The destination table, on the other hand, has still the state from before the update was applied. So a simple endpoint matching won't really work. You need to track former way states and deleted ways as well. And that is somewhat tricky with osm2pgsql.

The tile expiry kindly solves the problem for you because it does the expiry always on the old and the new geometry. Thus, the expired region cover where ways were before and after the update, nicely side-stepping the issue at the price of invalidating a bit more than necessary.

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