ST_ExteriorRing

Introduction: Returns a line string representing the exterior ring of the POLYGON geometry. Return NULL if the geometry is not a polygon.

Format: ST_ExteriorRing(geom: geometry)

SQL example:

SELECT ST_ExteriorRing(ST_GeomFromText('POLYGON((0 0 1, 1 1 1, 1 2 1, 1 1 1, 0 0 1))'))

Output: LINESTRING (0 0, 1 1, 1 2, 1 1, 0 0)