ST_Multi — Devuelve la geometría como una geometría MULTI*.
geometry ST_Multi(geometry geom);
Returns the geometry as a MULTI* geometry collection. If the geometry is already a collection, it is returned unchanged.
A Polygon is converted to a single-element MultiPolygon. The coordinates are unchanged.
SELECT ST_Multi('POLYGON ((10 30,30 30,30 10,10 10,10 30))');
MULTIPOLYGON(((10 30,30 30,30 10,10 10,10 30)))