Add highway names

layers/highway_name/highway_name.yaml

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+layer:
+  id: "highway_name"
+  description: |
+      [OpenStreetMap highway] which is used for roads, paths and cycletracks
+      and other recognised routes on land.
+  buffer_size: 8
+  srs: +proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0.0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs +over
+  fields:
+    class: String
+  datasource:
+    geometry_field: geometry
+    srid: 900913
+    query: (SELECT osm_id, geometry, name, class::text, subclass FROM layer_highway_name(!bbox!, z(!scale_denominator!))) AS t
+schema:
+  - ./merge_highways.sql
+  - ./layer.sql
+datasources:
+  - type: imposm3
+    mapping_file: ../highway/mapping.yaml

layers/highway_name/layer.sql

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+CREATE OR REPLACE FUNCTION layer_highway_name(bbox geometry, zoom_level integer)
+RETURNS TABLE(osm_id bigint, geometry geometry, name text, class highway_class, subclass text) AS $$
+    SELECT osm_id, geometry, name, to_highway_class(highway) AS class, highway AS subclass FROM (
+        SELECT * FROM osm_highway_name_linestring
+        WHERE zoom_level = 12 AND to_highway_class(highway) < 'minor_road'::highway_class AND NOT highway_is_link(highway)
+        UNION ALL
+        SELECT * FROM osm_highway_name_linestring
+        WHERE zoom_level = 13 AND to_highway_class(highway) < 'path'::highway_class
+        UNION ALL
+        SELECT * FROM osm_highway_name_linestring
+        WHERE zoom_level >= 14
+    ) AS zoom_levels
+    WHERE geometry && bbox
+    ORDER BY z_order ASC;
+$$ LANGUAGE SQL IMMUTABLE;

layers/highway_name/merge_highways.sql

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+-- Instead of using relations to find out the road names we
+-- stitch together the touching ways with the same name
+-- to allow for nice label rendering
+-- Because this works well for roads that do not have relations as well
+CREATE TABLE IF NOT EXISTS osm_highway_name_linestring AS (
+	SELECT
+		(ST_Dump(geometry)).geom AS geometry,
+        -- NOTE: The osm_id is no longer the original one which can make it difficult
+        -- to lookup road names by OSM ID
+		member_osm_ids[0] AS osm_id,
+		member_osm_ids,
+		name,
+		highway,
+		z_order
+	FROM (
+		SELECT
+			ST_LineMerge(ST_Union(geometry)) AS geometry,
+			name,
+			highway,
+			min(z_order) AS z_order,
+			array_agg(DISTINCT osm_id) AS member_osm_ids
+	    FROM osm_highway_linestring
+        -- We only care about roads for labelling
+	    WHERE name <> ''
+	    GROUP BY name, highway
+	) AS highway_union
+);
+
+CREATE INDEX IF NOT EXISTS osm_highway_name_linestring_geometry_idx ON osm_important_place_point USING gist(geometry);