Move to new modular layer structure

layers/urban/urban.sql

@@ -0,0 +1,30 @@
+CREATE OR REPLACE VIEW urban_z4 AS (
+    SELECT geom, scalerank
+    FROM ne_50m_urban_areas
+    WHERE scalerank <= 2
+);
+
+CREATE OR REPLACE VIEW urban_z5 AS (
+    SELECT geom, scalerank
+    FROM ne_50m_urban_areas
+);
+
+CREATE OR REPLACE VIEW urban_z6 AS (
+    SELECT geom, scalerank
+    FROM ne_10m_urban_areas
+);
+
+CREATE OR REPLACE FUNCTION layer_urban(bbox geometry, zoom_level int)
+RETURNS TABLE(geom geometry, scalerank int) AS $$
+    SELECT geom, scalerank FROM (
+        SELECT * FROM urban_z4
+        WHERE zoom_level = 4
+        UNION ALL
+        SELECT * FROM urban_z5
+        WHERE zoom_level = 5
+        UNION ALL
+        SELECT * FROM urban_z6
+        WHERE zoom_level BETWEEN 6 AND 10 AND scalerank-1 <= zoom_level
+    ) AS zoom_levels
+    WHERE geom && bbox;
+$$ LANGUAGE SQL IMMUTABLE;