sql - 迭代并单独编写线串(generate_series)

Question

我有一张混合了多边形和多边形的表格，我想运行一个函数将它们分解为 LineString(或 MultiLinestring)。

我的问题 - 我目前正在返回一个集合......没有几何(？)......

函数目前看起来像这样...(最终基于 Mike T. 帮助)

CREATE OR REPLACE FUNCTION LinesFromPoly2(polygon geometry) 
RETURNS SETOF geometry_dump AS 
$BODY$DECLARE 
 m integer; 
 g geometry;
 p geometry_dump%ROWTYPE; 
BEGIN
    FOR m IN SELECT generate_series(1, ST_NumGeometries($1)) LOOP 
      p.path[1] := m;
      p.geom := ST_Boundary(ST_GeometryN($1, m));
     RETURN NEXT p;
     END LOOP;
  RETURN; 
END;$BODY$
LANGUAGE plpgsql ;

电话:

SELECT id, name, LinesFromPoly2(the_geom)
  FROM public.poly_and_multipoly;

返回:

1|A|({1},)
2|B|({1},)
2|B|({2},)

示例数据:

CREATE TABLE poly_and_multipoly (
  "id" SERIAL NOT NULL PRIMARY KEY,
  "name" char(1) NOT NULL,
  "the_geom" geometry NOT NULL
);
-- add data, A is a polygon, B is a multipolygon
INSERT INTO poly_and_multipoly (name, the_geom) VALUES (
    'A', 'POLYGON((7.7 3.8,7.7 5.8,9.0 5.8,7.7 3.8))'::geometry
    ), (
    'B',
    'MULTIPOLYGON(((0 0,4 0,4 4,0 4,0 0),(1 1,2 1,2 2,1 2,1 1)), ((-1 -1,-1 -2,-2 -2,-2 -1,-1 -1)))'::geometry
);

Answer 1

你不需要自定义函数来做你想做的事。例如，尝试只访问 ST_Dump 的两个成员(路径 和geom):

SELECT id, name,
  (ST_Dump(ST_Boundary(the_geom))).path[1],
  ST_AsText((ST_Dump(ST_Boundary(the_geom))).geom)
FROM poly_and_multipoly;

 id | name | path |                 st_astext
----+------+------+-------------------------------------------
  1 | A    |      | LINESTRING(7.7 3.8,7.7 5.8,9 5.8,7.7 3.8)
  2 | B    |    1 | LINESTRING(0 0,4 0,4 4,0 4,0 0)
  2 | B    |    2 | LINESTRING(1 1,2 1,2 2,1 2,1 1)
  2 | B    |    3 | LINESTRING(-1 -1,-1 -2,-2 -2,-2 -1,-1 -1)
(4 rows)

或者每个几何部分一个 [MULTI]LINESTRING:

SELECT id, name,
  (ST_Dump(the_geom)).path[1],
  ST_AsText(ST_Boundary((ST_Dump(the_geom)).geom))
FROM poly_and_multipoly;

 id | name | path |                          st_astext
----+------+------+--------------------------------------------------------------
  1 | A    |      | LINESTRING(7.7 3.8,7.7 5.8,9 5.8,7.7 3.8)
  2 | B    |    1 | MULTILINESTRING((0 0,4 0,4 4,0 4,0 0),(1 1,2 1,2 2,1 2,1 1))
  2 | B    |    2 | LINESTRING(-1 -1,-1 -2,-2 -2,-2 -1,-1 -1)
(3 rows)

但如果您确实想使用 LinesFromPolygon2 函数，解决方法很简单:将几何体分配给 p.geom，而不是 g ，即

p.geom := ST_Boundary(ST_GeometryN($1, m));