ST_Curve If the geometry-expression is of type ST_Curve, return the geometry-expression. If the geometry-expression is a geometry collection with a single element of type ST_Curve, return that element. If the geometry-expression is the empty set, return an empty set of type ST_LineString. Otherwise, raise an exception condition.
Examples
Example 1 –
The following example returns the result LineString (0 0, 1 1, 2 0).
SELECT NEW ST_GeomCollection( 'GeometryCollection(LineString(0 0, 1 1, 2 0))' ).ST_ToCurve()