Move to new modular layer structure

layers/ice/ice.sql

@@ -0,0 +1,30 @@
+CREATE OR REPLACE VIEW ice_z0 AS (
+    SELECT geom, 'glacier' AS type FROM ne_110m_glaciated_areas
+);
+
+CREATE OR REPLACE VIEW ice_z2 AS (
+    SELECT geom, 'glacier' AS type FROM ne_50m_glaciated_areas
+    UNION ALL
+    SELECT geom, 'ice_shelf' AS type FROM ne_50m_antarctic_ice_shelves_polys
+);
+
+CREATE OR REPLACE VIEW ice_z5 AS (
+    SELECT geom, 'glacier' AS type FROM ne_10m_glaciated_areas
+    UNION ALL
+    SELECT geom, 'ice_shelf' AS type FROM ne_10m_antarctic_ice_shelves_polys
+);
+
+CREATE OR REPLACE FUNCTION layer_ice(bbox geometry, zoom_level int)
+RETURNS TABLE(geom geometry, class text) AS $$
+    SELECT geom, type::text AS class FROM (
+        SELECT geom, type FROM ice_z0
+        WHERE zoom_level BETWEEN 0 AND 1
+        UNION ALL
+        SELECT * FROM ice_z2
+        WHERE zoom_level BETWEEN 2 AND 4
+        UNION ALL
+        SELECT * FROM ice_z5
+        WHERE zoom_level BETWEEN 5 AND 8
+    ) AS zoom_levels
+    WHERE geom && bbox;
+$$ LANGUAGE SQL IMMUTABLE;