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ST_Boundary

Returns the closure of the combinatorial boundary of a geometry.

Syntax​

ST_Boundary(geometry)

Parameters​

ParameterTypeDescription
geometryGeometryInput geometry

Returns​

TypeDescription
Binary (WKB)Boundary geometry as WKB with geoarrow.wkb metadata

Examples​

Polygon Boundary​

SELECT ST_Boundary(ST_GeomFromText('POLYGON((0 0, 4 0, 4 4, 0 4, 0 0))'));
-- Returns: LINESTRING(0 0, 4 0, 4 4, 0 4, 0 0)

LineString Boundary​

SELECT ST_Boundary(ST_GeomFromText('LINESTRING(0 0, 10 10)'));
-- Returns: MULTIPOINT(0 0, 10 10) - the endpoints

Closed LineString Boundary​

SELECT ST_Boundary(ST_GeomFromText('LINESTRING(0 0, 4 0, 4 4, 0 0)'));
-- Returns: GEOMETRYCOLLECTION EMPTY - closed rings have empty boundary

Extract Polygon Rings​

SELECT id, ST_Boundary(geom) as outline
FROM parcels;

Notes​

  • Points return empty geometry (no boundary)
  • LineStrings return endpoints as MultiPoint (empty if closed)
  • Polygons return the ring(s) as LineString or MultiLineString
  • Boundary dimension is always one less than the input dimension
  • Useful for extracting polygon outlines

See Also​