ST_NumInteriorRings
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Name

ST_NumInteriorRings — Returns the number of interior rings (holes) of a Polygon.

Synopsis

integer ST_NumInteriorRings(geometry a_polygon);

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;