[postgis] How to indentify each polygons' donut polygons in multipolygon

Dave Blasby dblasby at refractions.net
Tue Dec 18 09:16:47 PST 2001

> Hisaji ONO wrote:
> Hi.
>   I've currently developed GML 2.0 convesion code from PostGIS's WKT.
>  I've known PostGIS's NumInteriorRings & InteriorRingN function can
> deal with interior rings in polygons. But InteriorRingN  function
> seemed to support "single" polygons.
>  How to identify whether each polygons within
> multipolygon have interior rings or not?

This is, unfortunately, a problem with the OGC spec - its difficult to
directly access anything in a collection (ie. MULTI* or

Use the GeometryN(geometry,integer) function to get a sub-geometry.  Ie.

NumInteriorRings(GeometryN(the_geom, 2))

To find out how many interior rings are in polygon #2.

(the_geom) must be a multi* or geometrycollection.


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