ST_Boundary

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

Format: ST_Boundary(geom: Geometry)

Since: v1.3.0

Example:

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

Output:

LINEARRING (1 1, 0 0, -1 1, 1 1)