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.
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)))'