This topic describes the ST_InteriorRingN function. This function returns the Nth interior ring of the input polygon object as a LineString object. The interior rings are identified by the specified indexes. The indexes start from 1.


geometry  ST_InteriorRingN(geometry  aPolygon , integer  n);


Parameter Description
aPolygon The polygon object that you want to specify.
n The index number of the interior ring that you want to obtain.


  • If the input geometry object is not a polygon object or the value of the n parameter is invalid, this function returns NULL.
  • This function does not support MultiPolygon objects. You need to use the ST_Dump function to convert a MultiPolygon object into a polygon object.
  • This function supports 3D objects and does not delete z coordinates.


SELECT ST_AsText(ST_InteriorRingN('POLYGON((1 0,3 0,0 3,1 0),(1 0,2 0, 0 2,1 0))'::geometry, 1));
 LINESTRING(1 0,2 0,0 2,1 0)
(1 row)