An ST_GeomCollection is a collection of zero or more ST_Geometry values.
All of the values are in the same spatial reference system as the collection value.
The ST_GeomCollection type can contain a heterogeneous collection of objects (for example, points, lines, and polygons).
Sub-types of ST_GeomCollection can be used to restrict the collection to certain geometry types.
The dimension of the geometry collection value is the largest dimension of its constituents.
A geometry collection is simple if all of the constituents are simple and no two constituent geometries
intersect except possibly at their boundaries.