ApsaraDB RDS:ST_ExteriorRing

Syntax

geometry ST_ExteriorRing(geometry aPolygon);

Parameters

Return type

geometry of subtype LineString.

• If aPolygon is not specified, the function returns NULL.

• Z coordinates are preserved. The function supports 3D geometries.

Examples

Basic example

Extract the exterior ring from a polygon with a hole:

SELECT ST_AsText(ST_ExteriorRing('POLYGON((1 0,3 0,0 3,1 0),(1 0,2 0,0 2,1 0))'::geometry));

Output:

          st_astext
-----------------------------
 LINESTRING(1 0,3 0,0 3,1 0)
(1 row)

The inner ring (hole) is ignored. Only the outer boundary is returned.

Query a table of polygons

SELECT gid, ST_ExteriorRing(geom) AS exterior_ring
FROM my_polygons;

3D polygon

Z coordinates are retained in the output:

SELECT ST_AsEWKT(
    ST_ExteriorRing(
        'POLYGON((0 0 1,1 1 1,1 2 1,0 0 1))'::geometry
    )
);

Output:

              st_asewkt
-------------------------------------
 LINESTRING(0 0 1,1 1 1,1 2 1,0 0 1)
(1 row)