[postgis-users] PostGIS 1.3.3 on Windows XP Unstable

[Martin Davis]

You're right about how the current PostGIS union works.

The CascadedUnion algorithm in JTS uses spatial indexing and some other 
heuristics to obtain very fast performance.

The Java2D approach that you give is similar to the buffer(0) trick that 
was the recommended way to do fast unioning prior to CascadedUnion.  
buffer(0) is sometime less stable, however, and it doesn't handle 
non-polygonal geometry.

Bresnahan, Mike wrote:
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>   
>> FWIW:  The unioning operations currently in PostGIS are very
>>     
> inefficient.
>
> Do I guess right that PostGIS iterates over each geometry and unions it
> with the result of all previous geometries, something like this:
>
>             java.awt.geom.Area area = null;
>             for (Geometry geom : geomList) {
>                 if (area == null) {
>                     area = ConvertToArea(geom);
>                 }
>                 else {
>                     path = area.add(ConvertToArea(geom)); // union
>                 }
>             }
>
> AFAIK, that is what Oracle does and it is quite slow.
>
>   
>> We are working on improving the union operation in PostGIS and
>>     
> hopefully
>   
>> will have at least a first draft to show in the next release.  Most
>> notably
>> porting the JTS logic for doing Cascaded Unions to PostGIS.
>>     
>
> By Cascaded Unions do you mean a routine that works something like this:
>
>             java.awt.geom.Area subArea = null; 
>             int rowCount = 0;
>             while (result.next()) {
>                 JGeometry geom = JGeometry.load(result.getBytes(1));
>                 java.awt.Shape shape =
> Java2DClipper.geometryToShape(geom);
>                 if (shape != null) {
>                     if (subArea == null) {
>                         subArea = new java.awt.geom.Area(shape);
>                     }
>                     else {
>                         subArea.add(new java.awt.geom.Area(shape));
>                     }
>                     ++rowCount;
>                     if (rowCount % 10 == 0) {
>                          if (area == null) {
>                             area = subArea;
>                         }
>                         else {
>                             area.add(subArea);
>                         }
>                         subArea = null;
>                     }
>                 }
>                 System.out.println(rowCount);
>             }
>
> I have found that this is indeed faster that the first approach, but
> what I have found that is even faster is to create one large geometry
> collection and union it with itself like this:
>
>             java.awt.geom.GeneralPath path = null;
>             for (Geometry geom : geomList) {
>                 if (path == null) {
>                     path = ConvertToPath(geom);
>                 }
>                 else {
>                     path = AppendToPath(path, geom);
>                 }
>             }
>             java.awt.geom.Area area = new java.awt.geom.Area(path);
>             area.add(area); // self-union
>
> I think the gain here is that you only build the temp data structures
> for each polygon once. I'm guessing that this is what ArcMap does.
>
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>   

-- 
Martin Davis
Senior Technical Architect
Refractions Research, Inc.
(250) 383-3022