ST_NumInteriorRings

Syntax

integer ST_NumInteriorRings(geometry aPolygon)

Parameters

Usage notes

Returns NULL if the input geometry is not a polygon.

Examples

Query interior ring count from a table

SELECT gid, ST_NumInteriorRings(geom) AS num_holes
FROM sometable;

Compare ST_NumInteriorRings and ST_NRings

ST_NRings counts all rings (exterior + interior), while ST_NumInteriorRings counts only interior rings (holes):

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

Output:

 st_nrings | st_numinteriorrings
-----------+---------------------
         2 |                   1
(1 row)

The polygon has 2 rings total (1 exterior + 1 interior), so ST_NRings returns 2 and ST_NumInteriorRings returns 1.

See also

• ST_NRings — returns the total number of rings (exterior + interior) for a polygon or MultiPolygon