ST_ForceCurve

Converts a geometry to its curved representation: lines become CompoundCurves, MultiLineStrings become MultiCurves, polygons become CurvePolygons, and MultiPolygons become MultiSurfaces. If the input is already in curved form, the function returns it unchanged.

This function supports 3D geometries and preserves Z coordinates. It also supports CircularStrings and curves.

geometry ST_ForceCurve(geometry g);

SELECT ST_AsText(ST_ForceCurve(ST_GeomFromText('POLYGON((1 1,2 1,2 2,1 2,1 1))')));

              st_astext
-------------------------------------
 CURVEPOLYGON((1 1,2 1,2 2,1 2,1 1))
(1 row)