ST_Boundary method for type ST

ST_Boundary method for type ST_Geometry

Returns the boundary of the geometry value.

Syntax

geometry-expression.ST_Boundary()

Returns

ST_Geometry Returns a geometry value representing the boundary of the geometry-expression.
The spatial reference system identifier of the result is the same as the spatial reference system of the geometry-expression.

The ST_Boundary method returns the spatial boundary of the geometry-expression. Geometries are characterized by their interior, boundary, and exterior. All geometry values are defined to be topologically closed, that is the boundary is considered to be part of the geometry.

Point geometries have an empty boundary. Curve geometries may be closed, in which case they have an empty boundary. If a curve is not closed, the start and end point of the curve form the boundary. For a surface geometry, the boundary is the set of curves that delineate the edge of the surface. For example, for a polygon the boundary of the geometry consists of the exterior ring and any interior rings.

Note

If the geometry-expression is an empty geometry (ST_IsEmpty()=1), then this method returns NULL.

Note

This method can not be used with geometries in round-Earth spatial reference system.

Standards and compatibility

Example

The following example construct a geometry collection containing a polygon and a linestring and returns the boundary for the collection. The returned boundary is a collection containing the exterior ring of the polygon and the two end points of the linestring. It is equivalent to the following collection: 'GeometryCollection (LineString (0 0, 3 0, 3 3, 0 3, 0 0), MultiPoint ((0 7), (4 4)))'

SELECT NEW ST_GeomCollection('GeometryCollection (Polygon ((0 0, 3 0, 3 3, 0 3, 0 0)), LineString (0 7, 0 4, 4 4))').ST_Boundary()

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