The boundary of an ST_Geometry value is a set of ST_Geometry values of the next lower dimension.
| IF the ST_Geometry represents … | THEN the boundary … |
|---|---|
| an ST_Point or ST_MulitPoint value | is the empty set. |
| a nonclosed ST_LineString or GeoSequence value | consists of the start and end ST_Point values. |
| a closed ST_LineString or GeoSequence value | is empty. |
| an ST_MultiLineString value | consists of ST_Point values that are in the boundaries of an odd number of its element ST_LineString values. |
| an ST_Polygon value | consists of its set of linear rings. |
| an ST_MultiPolygon value | consists of the set of linear rings of its ST_Polygon values. |
| an ST_GeomCollection where the interiors of the geometries in the collection are disjoint | consists of geometry values drawn from the boundaries of the element geometries, where the geometry values are in the boundaries of an odd number of element geometries. |