ST_NRings

Syntax

integer ST_NRings(geometry geomA)

Parameters

Usage notes

• Unlike ST_NumInteriorRings, ST_NRings counts both the exterior ring and all interior rings. For a simple polygon with no holes, ST_NRings returns 1 and ST_NumInteriorRings returns 0.

• Supports 3D geometries. Z coordinates are preserved.

• Supports circular strings and curves.

Examples

The following query runs both functions on the same geometry, showing the difference between total ring count and interior ring count:

SELECT
    ST_NRings(geom)           AS nrings,
    ST_NumInteriorRings(geom) AS ninterior_rings
FROM (
    SELECT 'POLYGON((1 0,0 3,3 0,1 0),(1 0,2 0,0 2,1 0))'::geometry AS geom
) AS t;

 nrings | ninterior_rings
--------+-----------------
      2 |               1
(1 row)

The polygon has one exterior ring and one interior ring (hole), so ST_NRings returns 2 while ST_NumInteriorRings returns 1.

For a polygon with no interior rings:

SELECT ST_NRings('POLYGON((1 0,0 3,3 0,1 0))'::geometry);

 st_nrings
-----------
         1
(1 row)

See also

• ST_NumInteriorRings