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)))'