名称
ST_DumpRings — 返回多边形外环和内环的一组geometry_dump行。
大纲
geometry_dump[] ST_DumpRings(geometry a_polygon);
描述
提取多边形环的集合返回函数 (SRF)。它返回一组geometry_dump 行，每行包含一个几何图形（geom字段）和一个整数数组（path 字段）。
geom 字段将每个环包含为 POLYGON。path字段是一个长度为 1 的整数数组，包含多边形环索引。外环（壳）的索引为 0。内环（孔）的索引为 1 及更高。
![[注意]](../images/note.png)
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This only works for POLYGON geometries, not MULTIPOLYGONs. Use ST_Dump to extract polygon elements from polygonal geometries: |
可用性：需要 PostGIS 1.1.3 PostgreSQL 7.3 或更高版本。
该函数支持 3d 并且不会丢失 z-index。
示例
将所有环提取为 polygon。
Code
SELECT polyTable.field1, polyTable.field1,
(ST_DumpRings(polyTable.geom)).geom As geom
FROM polyTable
从 polygon 中提取外壳（shell）和洞（holes）。
Code
SELECT path,geom As geom
FROM ST_DumpRings('POLYGON ((1 9,9 9,9 1,1 1,1 9),(2 2,2 3,3 3,3 2,2 2),(4 2,4 4,6 4,6 2,4 2))');
栅格输出
path | geom
------+--------------------------------
{0} | POLYGON((1 9,9 9,9 1,1 1,1 9))
{1} | POLYGON((2 2,2 3,3 3,3 2,2 2))
{2} | POLYGON((4 2,4 4,6 4,6 2,4 2))
Figure
Finding interior rings which lie close to the exterior ring.
Code
WITH poly AS (
SELECT 'POLYGON ((
0 0, 10 0, 10 10, 0 10, 0 0
), (
1 1, 1 2, 2 2, 2 1, 1 1
), (
8.5 1, 8.5 2, 9.5 2, 9.5 1, 8.5 1
))'::geometry AS geom
),
rings AS (
SELECT d.path[1] AS ring_no,
ST_Boundary(d.geom) AS ring_geom
FROM poly
CROSS JOIN LATERAL ST_DumpRings(poly.geom) AS d
),
shell AS (
SELECT ring_geom
FROM rings
WHERE ring_no = 0
),
holes AS (
SELECT ring_no, ring_geom
FROM rings
WHERE ring_no > 0
)
SELECT h.ring_no,
ST_Normalize(ST_Multi(ST_LineMerge(
ST_Intersection(ST_Buffer(s.ring_geom, 1.0), h.ring_geom)
))) AS close_part
FROM shell AS s
JOIN holes AS h
ON ST_DWithin(s.ring_geom, h.ring_geom, 1.0);
栅格输出
ring_no | close_part
---------+-------------------------------------------------
1 | MULTILINESTRING((1 2,1 1,2 1))
2 | MULTILINESTRING((8.5 1,9 1,9.5 1,9.5 2,9 2))
Figure