[Tile-serving] [osm2pgsql-dev/osm2pgsql] Transform all estimated extents to EPSG:3857 (PR #2498)

[Jochen Topf]

@joto commented on this pull request.

I have two problems with this:

a) Everything working with tiles in osm2pgsql assumes 3857 and we don't guarantee that it works with anything else. Presumably you have tested this in your case, but I think we still need to figure out what the guarantees of osm2pgsql are here and we should have tests to make sure we keep those guarantees.
b) It is not obvious that the implementation here is correct for all cases. We could have several geometries with different SRIDs for instance (although I have never seen that). Or no `extent` is set for the raster columns (which is currently the case in a database I have on hand here). It all hinges on obscure settings in the `raster_columns` and `geometry_columns` views and the behaviour of the `ST_EstimatedExtent()` function.

I am not saying this is wrong, I am just not sure we are understanding all the ramifications well enough. And maybe we can improve the implementation. We can run more than one query for instance, first getting the SRID then doing something else or so. That might make this more readable than putting everything into the one query. And it would give us the chance of generating better warning/error messages.

>              schema, table, column);
     } else {
         result = db_connection.exec(
+            "WITH srid AS ("
+            "  SELECT ST_SRID({}) AS srid FROM {} LIMIT 1"

The column and table name should be quoted.

`ST_SRID("{}") AS srid FROM "{}" LIMIT 1`

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