ST_Boundary

Introduction: Returns the closure of the combinatorial boundary of this Geometry.

Format: ST_Boundary(geom: geometry)

SQL example:

SELECT ST_Boundary(ST_GeomFromText('POLYGON((1 1,0 0, -1 1, 1 1))'))

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