[postgis-users] PostGIS 1.3.3 on Windows XP Unstable

Thu Aug 14 16:45:10 PDT 2008

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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.