-- An ornithologist, wishing to study the bird population on several south
-- sea islands, knows that the feeding zone of the bird species she is
-- interested in is restricted to the shoreline. As part of her calculation
-- of the island's carrying capacity the ornithologist requires the islands'
-- perimeters. Some of the islands are so large they have several ponds on
-- them. However, the shoreline of the ponds are inhabited exclusively by
-- another more aggressive bird species. Therefore, the ornithologist
-- requires the perimeter of the exterior ring only of the islands.
-- The ID and name columns of the islands table identifies each island,
-- while the land polygon column stores the island's geometry.

CREATE TABLE islands (id    integer,
                      name  varchar(32),
                      land  ST_Polygon);

INSERT INTO islands VALUES(
   1,
   'hawaii',
   ST_PolyFromText('polygon ((42000 18000,48000 18000,48000 13000,42000 13000,42000 18000))',1000)
);

-- The ST_ExteriorRing function extracts the exterior ring from each
-- island polygon as a linestring. The length of the linestring is
-- calculated by the ST_Length function. The linestring lengths are
-- summarized by the sum function.

SELECT SUM(ST_Length(ST_ExteriorRing(land)))
  FROM islands;