PostGIS
Description
Return the number of interior rings of a polygon geometry. Return NULL if the geometry is not a polygon.
This method
implements the SQL/MM specification. SQL-MM 3: 8.2.5
Changed: 2.0.0 - in prior versions it would allow passing a MULTIPOLYGON, returning the number of interior rings of first POLYGON.
Examples
--If you have a regular polygon
SELECT gid, field1, field2, ST_NumInteriorRings(geom) AS numholes
FROM sometable;
--If you have multipolygons
--And you want to know the total number of interior rings in the MULTIPOLYGON
SELECT gid, field1, field2, SUM(ST_NumInteriorRings(geom)) AS numholes
FROM (SELECT gid, field1, field2, (ST_Dump(geom)).geom As geom
FROM sometable) As foo
GROUP BY gid, field1,field2;