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

[Jochen Topf]

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

> If performance is a problem, I know for sure I could make it much faster if the generalization could receive the exact ways that were edited, like carto's flex lua code, rather than Z/X/Y tiles. That way the slow part (the &&tile_box) can be skipped entirely, and the remaining steps would only walk the network of the actually edited way(s).

You don't need to base this on tiles, you can base this on the ways that changed! For instance, you can have a separate table where you store (the "way" processing function) all the ids of ways that changed, then use that table as input for the rest of the processing in the "gen" step. But it is not as easy as one would think to implement this. You have to correctly handle deleted ways and all the different kinds of change, including cases like this: Say you have two ways that are not connected, now a third way is changed which makes it connect to both existing and unchanged ways allowing all of them to be merged.

> so given multithreading this should be basically fine I'd hope

All of this is I/O bound so multithreading is not going to help.

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