[postgis-users] PostGIS 1.3.3 on Windows XP Unstable

[Paragon Corporation]

Martin,

I thought the below looks more like ST_MemUnion more than the ST_Union
approach no?   The ST_Union approach gets a full array of geometries and
then uses the unite_garray function to process them.   I assumed the
unite_garray function then does something a bit smarter than the below.
Although embarrasingly I have never stopped to look what unitegarray is
actually doing.  Its always nice to pass the buck to the other guy and
assume you are dealing with nice efficient black boxes.

Thanks,
Regina

-----Original Message-----
From: postgis-users-bounces at postgis.refractions.net
[mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of Martin
Davis
Sent: Thursday, August 14, 2008 8:12 PM
To: PostGIS Users Discussion
Subject: Re: [postgis-users] PostGIS 1.3.3 on Windows XP Unstable

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:
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA256
>
>   
>> 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

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