David Blasby wrote:
Jody,
I looked more into the featurewriting stuff that we talked about last
night -- here's what I found.
I think there's a problem in how "Feature" implements "equals". Here's
where it came up.
I wrote a tiny program that updates a table in my database:
/**
* similiar to:
* update <table name> set <destination geometry> =
generalize(<source geometry name>,<tolerance>);
*
* but the generalize() function in here is much "smarter" than the
one in postgis
...
*/
Here's my main loop (taken from the documentation page
"http://docs.codehaus.org/display/GEOTOOLS/Data+Writing";):
So we need the standard safety performance checks here ;-)
private void process() throws Exception
{
Transaction t = new DefaultTransaction("HACK");
FeatureWriter aWriter = ds.getFeatureWriter(table_name, t );
try{
while (aWriter.hasNext())
{
Feature f = aWriter.next(); // get a feature
Geometry g = (Geometry) f.getAttribute(source_column);
Geometry g_gen = generalize(g,tolerance); //
less point!!
f.setAttribute(dest_colum,g_gen); // modify it
aWriter.write(); // write it to DB
}
t.commit();
} finally {
aWriter.close();
}
}
Here's my problems.
1. This isnt happening in a transaction - it put my database into an
invalid state :(
See above! Make a test cases for Transcation.AUTO_COMMIT putting you
database into an invalid sate. Justin is the module maintainer
and his boss should be able to aerange for him to help you on this one :-)
2. DOCUMENTATION: its really weird seeing your writer acting as a
reader. Might want to emphasis that in the documentation. I put a
note at the top so others wouldnt be confused. You should also
include a brief summary of FeatureStore/FeatureSource/FeatureXYZ since
that is also confusing (ie. you ask for a FeatureSource, but you
really get a FeatureStore!).
This is covered in my document, however it has been shunted off to one
side in the user docs wiki space. I am not cleaning up the documentation
until the FM is done. I would be happy to clean up the javadocs, perhaps
with your example.
In general I would like to avoid FeatureReader and FeatureWriter. I do
understand that the situation you working from kind of needs it. You
should also be able to use an iterator() and the iterator will return
you clones, and if the clone is different from the original when next is
called the result will be written out.
I hope to do a better job of this for OracleDataStore, and would love to
see the changes picked up by postgis.
3. Unfortunately, my program doesnt work! As I mentioned, this has to
do with Feature.equals().
When the "aWriter.write();" occurs it gets passed off to the
JDBCFeatureWriter class which quickly does a:
live.equals(current) on line 193 (2.2.x)
in DefaultFeature (line 480) we see this:
NOTE: THIS CODE IS DUPLICATED IN
DataUtilities#attributesEqual()
if (!((Geometry) attributes[i]).equals(
(Geometry) otherAtt)) {
return false;
}
I dont think this is the correct behavior.
a) this takes a *long time* to compute
b) if your geometries are invalid, it will likely throw an
exception and you're screwed (thats what happened to me)
c) Geometry.equals(Geometry) probably doesnt do what you
think/want it to do.
For example, these 3 geometries are all equal:
LINESTRING(0 0, 10 10)
LINESTRING(0 0, 5 5, 10 10) // extra point that coincident
with the segment
LINESTRING(10 10,0 0) // backwards
NOTE: this means you cannot "flip" the direction of
lines in Geotools -- it'll never get written out.
The alternative is to use Geometry.equalsExact(Geometry). This is a
"relatively" quick method that checks to see if 2 geometries are the
same type, have the same number of points, and all the points are
equal and in the same order. Basically, what most people mean when
they say two geometries are equal.
If you want to make the change - cool. I would much rather see a
"isDirty" flag on the feature - and that is what I will do for oracle
Datastore. The thinking is if the attribute can remember it has been
changed then we can know the feature needs to be written without doing
any equals check.
So, how about changing all occurances of Geometry.equals(Geometry)
with Geometry.equalsExact(Geometry)? Is this going to cause problems?
I found these two places where it happens, but I'm sure there are others.
Dave if you make the change, and the test case passes please commit it
as a performance improvement. Since you are the first person to use
this seriously I would also love to have you around to do a review of my
oracle Datastore work. Even if that review is grabbing the design for
postgis.
Jody
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