union ocean regular squares into complex polygon (#1021)

The ocean layer (table osm_ocean_polygon insert by `make import-data`) is made from squares that have a 10m buffer. 

This creates extra geometry in vector tiles as:

Union of full ocean squares should decrease the size of the water layer.

there are 8042 polygons (squares, 5 points, more then 100km<sup>2</sup>), which was reduced to 22 polygons - 1184kB vs 40kB respectively. 

In mbtiles should be the size reduction even more significant, due to the creation of more polygons based on a 10m buffer of each square.

layers/water/update_water.sql

@@ -1,3 +1,27 @@
+-- Recreate ocean layer by union regular squares into larger polygons
+-- etldoc: osm_ocean_polygon -> osm_ocean_polygon_union
+CREATE TABLE IF NOT EXISTS osm_ocean_polygon_union AS
+    (
+    SELECT (ST_Dump(ST_Union(ST_MakeValid(geometry)))).geom::geometry(Polygon, 3857) AS geometry 
+    FROM osm_ocean_polygon
+    --for union select just full square (not big triangles)
+    WHERE ST_Area(geometry) > 100000000 AND 
+          ST_NPoints(geometry) = 5
+    UNION ALL
+    SELECT geometry 
+    FROM osm_ocean_polygon
+    -- as 321 records have less then 5 coordinates (triangle)
+    -- bigger then 5 coordinates have squares with holes from island and coastline
+    WHERE ST_NPoints(geometry) <> 5
+    );
+
+CREATE INDEX IF NOT EXISTS osm_ocean_polygon_union_geom_idx
+  ON osm_ocean_polygon_union
+  USING GIST (geometry);
+
+--Drop data from original table but keep table as `CREATE TABLE IF NOT EXISTS` still test if query is valid
+TRUNCATE TABLE osm_ocean_polygon;
+
 -- This statement can be deleted after the water importer image stops creating this object as a table
 DO
 $$
@@ -8,12 +32,12 @@ $$
     END;
 $$ LANGUAGE plpgsql;
 
--- etldoc: osm_ocean_polygon -> osm_ocean_polygon_gen1
+-- etldoc: osm_ocean_polygon_union -> osm_ocean_polygon_gen1
 DROP MATERIALIZED VIEW IF EXISTS osm_ocean_polygon_gen1 CASCADE;
 CREATE MATERIALIZED VIEW osm_ocean_polygon_gen1 AS
 (
 SELECT ST_Simplify(geometry, 20) AS geometry
-FROM osm_ocean_polygon
+FROM osm_ocean_polygon_union
     ) /* DELAY_MATERIALIZED_VIEW_CREATION */ ;
 CREATE INDEX IF NOT EXISTS osm_ocean_polygon_gen1_idx ON osm_ocean_polygon_gen1 USING gist (geometry);
 
@@ -28,12 +52,12 @@ $$
     END;
 $$ LANGUAGE plpgsql;
 
--- etldoc: osm_ocean_polygon -> osm_ocean_polygon_gen2
+-- etldoc: osm_ocean_polygon_union -> osm_ocean_polygon_gen2
 DROP MATERIALIZED VIEW IF EXISTS osm_ocean_polygon_gen2 CASCADE;
 CREATE MATERIALIZED VIEW osm_ocean_polygon_gen2 AS
 (
 SELECT ST_Simplify(geometry, 40) AS geometry
-FROM osm_ocean_polygon
+FROM osm_ocean_polygon_union
     ) /* DELAY_MATERIALIZED_VIEW_CREATION */ ;
 CREATE INDEX IF NOT EXISTS osm_ocean_polygon_gen2_idx ON osm_ocean_polygon_gen2 USING gist (geometry);
 
@@ -48,12 +72,12 @@ $$
     END;
 $$ LANGUAGE plpgsql;
 
--- etldoc: osm_ocean_polygon -> osm_ocean_polygon_gen3
+-- etldoc: osm_ocean_polygon_union -> osm_ocean_polygon_gen3
 DROP MATERIALIZED VIEW IF EXISTS osm_ocean_polygon_gen3 CASCADE;
 CREATE MATERIALIZED VIEW osm_ocean_polygon_gen3 AS
 (
 SELECT ST_Simplify(geometry, 80) AS geometry
-FROM osm_ocean_polygon
+FROM osm_ocean_polygon_union
     ) /* DELAY_MATERIALIZED_VIEW_CREATION */ ;
 CREATE INDEX IF NOT EXISTS osm_ocean_polygon_gen3_idx ON osm_ocean_polygon_gen3 USING gist (geometry);
 
@@ -68,11 +92,11 @@ $$
     END;
 $$ LANGUAGE plpgsql;
 
--- etldoc: osm_ocean_polygon -> osm_ocean_polygon_gen4
+-- etldoc: osm_ocean_polygon_union -> osm_ocean_polygon_gen4
 DROP MATERIALIZED VIEW IF EXISTS osm_ocean_polygon_gen4 CASCADE;
 CREATE MATERIALIZED VIEW osm_ocean_polygon_gen4 AS
 (
 SELECT ST_Simplify(geometry, 160) AS geometry
-FROM osm_ocean_polygon
+FROM osm_ocean_polygon_union
     ) /* DELAY_MATERIALIZED_VIEW_CREATION */ ;
 CREATE INDEX IF NOT EXISTS osm_ocean_polygon_gen4_idx ON osm_ocean_polygon_gen4 USING gist (geometry);

layers/water/water.sql

@@ -237,13 +237,13 @@ WHERE "natural" != 'bay'
 
 CREATE OR REPLACE VIEW water_z12 AS
 (
--- etldoc:  osm_ocean_polygon_gen1 ->  water_z12
+-- etldoc:  osm_ocean_polygon_union ->  water_z12
 SELECT geometry,
        'ocean'::text AS class,
        NULL::boolean AS is_intermittent,
        NULL::boolean AS is_bridge,
        NULL::boolean AS is_tunnel
-FROM osm_ocean_polygon
+FROM osm_ocean_polygon_union
 UNION ALL
 -- etldoc:  osm_water_polygon ->  water_z12
 SELECT geometry,
@@ -257,13 +257,13 @@ WHERE "natural" != 'bay'
 
 CREATE OR REPLACE VIEW water_z13 AS
 (
--- etldoc:  osm_ocean_polygon ->  water_z13
+-- etldoc:  osm_ocean_polygon_union ->  water_z13
 SELECT geometry,
        'ocean'::text AS class,
        NULL::boolean AS is_intermittent,
        NULL::boolean AS is_bridge,
        NULL::boolean AS is_tunnel
-FROM osm_ocean_polygon
+FROM osm_ocean_polygon_union
 UNION ALL
 -- etldoc:  osm_water_polygon ->  water_z13
 SELECT geometry,
@@ -277,13 +277,13 @@ WHERE "natural" != 'bay'
 
 CREATE OR REPLACE VIEW water_z14 AS
 (
--- etldoc:  osm_ocean_polygon ->  water_z14
+-- etldoc:  osm_ocean_polygon_union ->  water_z14
 SELECT geometry,
        'ocean'::text AS class,
        NULL::boolean AS is_intermittent,
        NULL::boolean AS is_bridge,
        NULL::boolean AS is_tunnel
-FROM osm_ocean_polygon
+FROM osm_ocean_polygon_union
 UNION ALL
 -- etldoc:  osm_water_polygon ->  water_z14
 SELECT geometry,