Refactor urban into table function

schema/layers/urban.sql

@@ -13,3 +13,19 @@ 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 $$
+    WITH zoom_levels AS (
+        SELECT * FROM urban_z4
+        WHERE zoom_level = 4
+        UNION ALL
+        SELECT * FROM urban_z5
+        WHERE zoom_level = 5
+        UNION ALL
+        SELECT * FROM urban_z6
+        WHERE zoom_level >= 6 AND scalerank-1 <= zoom_level
+    )
+    SELECT geom, scalerank FROM zoom_levels
+    WHERE geom && bbox;
+$$ LANGUAGE SQL IMMUTABLE;