Alessandro Radaelli wrote:
With geotools is it possible to query a Datastore with a query
containing aggregate functions like count(*) or sum(*) and group by
clauses? I mean the equivalent of an SQL statement like this
SELECT count(*), sex from population where ....spatial constraints...
GROUP BY sex
My datastore is Oracle, so i could issue the SQL directly, but I would
like to keep the Datastore abstraction, if possible...and if I perform
the "group by" on the resultset in my java code I have a very slow
solution and memory problems....
Support for aggregate functions is new -- I was just investigating this,
and found a bug, GEOT-797 (now fixed on trunk). Optimizations exist for
JDBC feature collections, so you should be able to do your aggregate
functions without iterating through the entire feature collection). I
don't know what will happen with multiple parameters + aggregates --
we'd need a good example to step through.
I've only tested this with PostGIS, but here is an example:
FeatureType featureType = dstore.getSchema("bc_voting_areas");
JDBCFeatureSource source = new JDBCFeatureSource(dstore, featureType);
Query query = new DefaultQuery(featureType.toString(), null);
FeatureCollection fc = new PostgisFeatureCollection(source, query);
//JDBCFeatureCollection could also be used here
ExpressionBuilder eb = new ExpressionBuilder();
FunctionExpression expr = (FunctionExpression)
eb.parser("Collection_Sum(vregist)");
int result = ((Number) expr.getValue(fc)).intValue();
http://svn.geotools.org/geotools/trunk/gt/plugin/postgis/test/org/geotools/data/postgis/collection/PostgisFeatureCollectionOnlineTest.java
There are likely better ways to do this (involving factories, etc), but
this one works.
Cheers,
Cory.
----
Cory Horner
Refractions Research Inc.
http://www.refractions.net
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