[Tile-serving] [osm2pgsql-dev/osm2pgsql] Linking between nodes and ways (Discussion #2455)

[dch0ph]

Thanks for `turning-circles.lua`. Some quick queries to make sure I've understood correctly:
- The `highways` table is just for this example, it could be the "regular" `planet_osm_line`?
- In the `highways_end` table, could this be "node-based" rather than way based? The logic of the final SQL query would be that of searching through nodes that are turning circles (rather than ways that contain TCs). 
- Is it necessary to pull out the tags of interest into `highways_end`, or can you address the way / node directly as in the SQL query above? Otherwise a change in "tags of interest" would require a re-import, which would be a big flexibility loss. Currently the pain point is making the spatial join, but, having done the join, all the tags are available.
- Am I right in thinking that the geometries are included in `highways_end` (rather than simply indices) to manage updates / expiry?
- Either way, would the node and way geometries be accessible to the SQL query? 

Connected to the point above, we would want to make the "joining tables" as general as possible to avoid a re-import. I think the database structure hasn't changed since hstore was introduced, and so we don't want to add features that reduce the current flexibility. There are a finite number of features that join nodes and ways (fords, turning circles, railway stations) and so it would make sense for these all to be imported whether or not the connection is used in the styling. Essentially, could `turning_circles` be `nodes_needing_ways` and `highways_end` be `nodes_attached_to_ways` and the rest be handled by the stylesheet?

 **Often you need the direction of the way to render the node in some special way, say a barrier that should be rendered perpendicular to the road**.

Yes, this would definitely be a useful special case. If  remember correctly, the French Carto style orients zebra crossings in this way via a [nasty looking SQL query](https://github.com/cquest/osmfr-cartocss/blob/9a7543eaee34fbb551e45550f3a14a7473639b17/osmfr.yml#L1083) involving a spatial join.
 
Other applications I've seen are orienting mountain pass symbols to the road direction.


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