Polygon decomposition

There are two levels of polygon geometry decomposition: they can be broken down either into LineStrings or into points. Decomposition into points is very similar to decomposition of a LineString: the ST_DumpPoints is used:

  SELECT ST_DumpPoints(
    ST_MakePolygon(
        ST_MakeLine(ARRAY[ST_MakePoint(20,50),ST_MakePoint(19.95,49.98), ST_MakePoint(19.90,49.90),ST_MakePoint(20,50)]),
        ARRAY[ST_MakeLine(ARRAY[ST_MakePoint(19.95,49.97),ST_MakePoint(19.943,49.963), ST_MakePoint(19.955,49.965),ST_MakePoint(19.95,49.97)])]
    )
);

                    st_dumppoints                     
------------------------------------------------------
 ("{1,1}",010100000000000000000034400000000000004940)
 ("{1,2}",01010000003333333333F333403D0AD7A370FD4840)
 ("{1,3}",01010000006666666666E633403333333333F34840)
 ("{1,4}",010100000000000000000034400000000000004940)
 ("{2,1}",01010000003333333333F333405C8FC2F528FC4840)
 ("{2,2}",0101000000C520B07268F133402506819543FB4840)
 ("{2,3}",010100000014AE47E17AF43340EC51B81E85FB4840)
 ("{2,4}",01010000003333333333F333405C8FC2F528FC4840)  

There's a slight difference in the geometry dump format: the path array now consists of two numbers. The first one is the index of the ring (1 being the exterior ring, and the following numbers the subsequent interior rings), and the second is the index a of point in the ring.

For extracting whole rings, PostGIS offers the following possibilities:

• Dumping the rings into a set of rows. For that, ST_DumpRings is used:

       SELECT ST_DumpRings(
       ST_MakePolygon(                     
       ST_MakeLine(ARRAY[ST_MakePoint(20,50),ST_MakePoint(19.95,49.98),    
       ST_MakePoint(19.90,49.90),ST_MakePoint(20,50)]),
       
       ARRAY[ST_MakeLine(ARRAY[ST_MakePoint(19.95,49.97),ST_MakePoint
       (19.943,49.963),  
       ST_MakePoint(19.955,49.965),ST_MakePoint(19.95,49.97)])]
        )                   
        );           
                              
              st_dumprings                                   
       ----------------------------------
      ({1},01030000000100000004000000(...) 

• Extracting the exterior ring:

       SELECT ST_ExteriorRing(
       ST_MakePolygon(         
       ST_MakeLine(ARRAY[ST_MakePoint(20,50),ST_MakePoint(19.95,49.98),  
       ST_MakePoint(19.90,49.90),ST_MakePoint(20,50)]),
      
       ARRAY[ST_MakeLine(ARRAY[ST_MakePoint(19.95,49.97),
       ST_MakePoint(19.943,49.963),ST_MakePoint(19.955,49.965),
       ST_MakePoint(19.95,49.97)])]
       )
       );

             st_exteriorring                        
       --------------------------
       01020000000400000000(...) 

• Extracting the interior rings based on their index:

        SELECT ST_InteriorRingN(
        ST_MakePolygon(
        ST_MakeLine(ARRAY[ST_MakePoint(20,50),ST_MakePoint(19.95,49.98),  
        ST_MakePoint(19.90,49.90),ST_MakePoint(20,50)]),         

        ARRAY[ST_MakeLine(ARRAY[ST_MakePoint(19.95,49.97),
        ST_MakePoint(19.943,49.963),        
        ST_MakePoint(19.955,49.965),ST_MakePoint(19.95,49.97)])]
         ), 1
        );

             st_interiorringn                       
        ---------------------------
        010200000004000000333(...)