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

[Dave Blasby]

> 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
GEOMETRYCOLLECTION).

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.

dave

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