Find a possible frontier between two polygons that intersect in several points in PostGIS

pg_frontier.sql

CREATE OR REPLACE FUNCTION frontiere(in poly_a geometry, in poly_b geometry) RETURNS geometry AS $frontiere$
DECLARE
	borders_intersection geometry;
	working_geom geometry;

	pct_debut float;
	pct_fin float;
	curs refcursor;
	pt_row record;
	prev_abs float;
	dist float;
	max_dist float; 
BEGIN
	IF (geometrytype(poly_a) <> 'POLYGON') OR (geometrytype(poly_b) <> 'POLYGON') THEN
		RETURN NULL;
	END IF;
	working_geom := ST_ExteriorRing(poly_a);
	borders_intersection := ST_Intersection (working_geom, ST_ExteriorRing(poly_b));

	CREATE TEMPORARY  TABLE ordered_points AS 
		SELECT pt, ST_Line_Locate_Point(working_geom, pt) AS abs FROM 
			( SELECT (ST_DumpPoints(borders_intersection)).geom AS pt) AS sub 
		ORDER BY ST_Line_Locate_Point(working_geom, pt) ;
	-- on considère que la portion la plus grande du polygone entre deux points d'intersection avec l'autre
	-- constitue l'extérieur. C'est pour celà qu'on recherche cette portion
	OPEN curs FOR SELECT * FROM ordered_points;
	FETCH LAST FROM curs INTO pt_row;
	prev_abs := pt_row.abs - 1;
	MOVE ABSOLUTE 0 FROM curs;
	max_dist := -1;
	LOOP
		FETCH curs INTO pt_row;
		EXIT WHEN NOT FOUND;
 
		dist := pt_row.abs - prev_abs;
		IF (dist > max_dist) THEN
			max_dist := dist;
			pct_debut := pt_row.abs;
			pct_fin := prev_abs;
		END IF;
		prev_abs := pt_row.abs;
	END LOOP;
	CLOSE curs;
	DROP TABLE IF EXISTS ordered_points;
	IF pct_fin < 0 THEN
		pct_fin := pct_fin + 1;
	END IF;
	IF pct_debut > pct_fin THEN
		RETURN ST_LineMerge(ST_Union(ST_Line_Substring(working_geom, pct_debut, 1), ST_Line_Substring(working_geom, 0, pct_fin)));
	ELSE
		RETURN ST_Line_Substring(working_geom, pct_debut, pct_fin);
	END IF;
END;
$frontiere$ LANGUAGE 'plpgsql';