Returns the nth geometry in the geometry collection.
SELECT NEW ST_GeomCollection('GeometryCollection (CircularString (5 10, 10 12, 15 10), Polygon ((10 -5, 15 5, 5 5, 10 -5)))') .ST_GeometryN( 2 )
SQL/MM (ISO/IEC 13249-3: 2006) 9.1.5