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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