ST_InteriorRingN — Returns the Nth interior ring (hole) of a Polygon.


geometry ST_InteriorRingN(geometry a_polygon, integer n);


Returns the Nth interior ring (hole) of a POLYGON geometry as a LINESTRING. The index starts at 1. Returns NULL if the geometry is not a polygon or the index is out of range.


This function does not support MULTIPOLYGONs. For MULTIPOLYGONs use in conjunction with ST_GeometryN or ST_Dump

This method implements the OGC Simple Features Implementation Specification for SQL 1.1.

This method implements the SQL/MM specification. SQL-MM 3: 8.2.6, 8.3.5

This function supports 3d and will not drop the z-index.


SELECT ST_AsText(ST_InteriorRingN(geom, 1)) As geom
		ST_Collect(ST_Buffer(ST_Point(1,2), 20,3),
			ST_Buffer(ST_Point(1, 2), 10,3))) As geom
		)  as foo;

See Also

ST_ExteriorRing, ST_BuildArea, ST_Collect, ST_Dump, ST_NumInteriorRing, ST_NumInteriorRings