Name
ST_NumInteriorRings — 返回多边形的内环（孔）数。
Synopsis
integer ST_NumInteriorRings(geometry a_polygon);
描述
返回多边形几何体的内环数。 如果几何图形不是多边形，则返回 NULL。
该方法实现了SQL/MM规范。 SQL-MM 3: 8.2.5
更改：2.0.0 - 在之前的版本中，它允许传递 MULTIPOLYGON，返回第一个 POLYGON 的内环数量。
示例
--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;