Move to new modular layer structure

layers/rail/rail.sql

@@ -0,0 +1,18 @@
+CREATE OR REPLACE VIEW rail_z13 AS (
+    SELECT * FROM rail
+    WHERE class='rail'
+);
+
+CREATE OR REPLACE VIEW rail_z14 AS (
+    SELECT * FROM rail
+);
+
+CREATE OR REPLACE FUNCTION layer_rail(bbox geometry, zoom_level int)
+RETURNS TABLE(geom geometry, class text, brunnel text) AS $$
+    SELECT way AS geom, class::text, brunnel::text FROM (
+        SELECT * FROM rail_z13 WHERE zoom_level = 13
+        UNION ALL
+        SELECT * FROM rail_z14 WHERE zoom_level >= 14
+    ) AS zoom_levels
+    WHERE way && bbox;
+$$ LANGUAGE SQL IMMUTABLE;