Get the lines between all points of a polygon in PostGis : avoid nested loop?

Question

I am trying to get all the lines between the points of a polygon, by writing PostGreSQL functions. I have written the two following functions that do the job fine :

• the first function to get all the points from the polygon

CREATE OR REPLACE FUNCTION PointsFromPolygon(polygon geometry) 
RETURNS SETOF geometry AS
$$
DECLARE
 point geometry;
BEGIN
FOR point IN SELECT DISTINCT points.geom FROM ( SELECT (ST_DumpPoints(polygon)).* ) AS points LOOP
 RETURN NEXT point;
END LOOP;
END;
$$
LANGUAGE plpgsql ;

• the second function to create all segments between these points.

CREATE OR REPLACE FUNCTION AllSegmentsFromPoints(polygon geometry)
RETURNS SETOF geometry AS
$$
DECLARE
 point1 geometry;
 point2 geometry;
 i integer;
BEGIN
i:=1;
 FOR point1 IN SELECT * FROM PointsFromPolygon(polygon) LOOP
 FOR point2 IN SELECT * FROM PointsFromPolygon(polygon) OFFSET i LOOP
 RETURN NEXT ST_MakeLine(point1,point2);
 END LOOP;
 i:= i+1;
 END LOOP;
END;
$$
LANGUAGE plpgsql;

Is there a better way to perform this ? Perhaps by doing a cartesian product between the points ?

Answer 1

I think I found my answer :

 WITH poly_geom1 AS (SELECT way FROM planet_osm_polygon WHERE id=1),
 poly_geom2 AS (SELECT way FROM planet_osm_polygon WHERE id=1),
 points1 AS (SELECT (ST_DumpPoints(poly_geom1.way)).* FROM poly_geom1),
 points2 AS (SELECT (ST_DumpPoints(poly_geom2.way)).* FROM poly_geom2)
 SELECT DISTINCT ST_MakeLine(points1.geom, points2.geom)
 FROM points1,points2
 WHERE points1.path <> points2.path;

Execution time for a 40 points polygon goes down from 135ms to 115ms.

Update

Thank to simplexio, the following query does exactly the same and is shorter :

WITH poly_geom AS (SELECT way FROM planet_osm_polygon WHERE id=1),
 points1 AS (SELECT (ST_DumpPoints(poly_geom.way)).* FROM poly_geom),
 points2 AS (SELECT (ST_DumpPoints(poly_geom.way)).* FROM poly_geom)
SELECT DISTINCT ST_MakeLine(points1.geom, points2.geom)
FROM points1,points2
WHERE points1.path <> points2.path;

• simpleuser001: can you drop poly_geom2 and use only poly_geom1 in points2 selection ? Just intrested.
• FredB: Yes you are right, I can use only poly_geom1. The execution time is exactly the same but the query shorter

Answer 2

Another solution, that avoids duplicate lines:

WITH poly(id, geom)AS (VALUES
 ( 1, 'POLYGON ((2 7, 5 9, 9 7, 8 3, 5 2, 2 3, 3 5, 2 7))'::geometry )
 )
,ring AS (SELECT ST_ExteriorRing(geom) geom FROM poly)
,pts AS (SELECT i, ST_PointN(geom, i) AS geom
 FROM ring
 JOIN LATERAL generate_series(2, ST_NumPoints(ring.geom)) AS s(i) ON true)
SELECT ST_MakeLine(p1.geom, p2.geom) AS geom
FROM pts p1, pts p2
WHERE p1.i > p2.i;