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

[leijurv]

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

I really appreciate you running it on a full planet.osm! That helps a lot, thank you!

You are correct. It was unintuitive but you were right, this can be done completely in Lua. It's the same SQL as it was in C++.

The only reason why I filtered to `name is not null or ref is not null` was just because the usecase is https://github.com/openstreetmap-carto/openstreetmap-carto/issues/951 which is about shields and labels. If there is no name and no ref, then there is no label and no shield, so no reason to waste the compute on merging. The idea is that unlabeled highway=yes doesn't benefit from linemerge - it draws almost identically either way (only off by a few pixels at most). You can see the usecase [here](https://github.com/leijurv/openstreetmap-carto/commit/b79c0fa3033ed5339643eb1b9590edc7e684dd28#diff-4d3b9fc913e4b3b031e5d24a39445393636f8ca254ff48ac1e579d83f2d569ffL1938-R1938): we only query from the `planet_osm_coalesced_roads` without any union. We essentially get a partial index on "has a label or shield" for free.

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