Hello,
I am trying to run a distinct values query, which should be at the same
time sorted and paged (offset and limit). I have tried the following code
(simplified):
Query query = new Query(typeName, Filter.INCLUDE, new String[]
{fieldName});
query.setSortBy(new SortBy[]{sortBy});
query.setStartIndex(startIndex);
query.setMaxFeatures(maxFeatures);
SimpleFeatureCollection featCol = featureSource.getFeatures(query);
Expression expr = factory.property(fieldName);
UniqueVisitor visitor = new UniqueVisitor(expr);
featCol.accepts(visitor, null);
I have have correctly understood the documentation, the feature collection
is not changed by the visitor, but I can get the result by using:
List uniqueValues = visitor.getResult().toList();
However, this result is not ordered, thus making impossible to run queries
such as:
SELECT DISTINCT(name) FROM osmadmlv2
ORDER BY name asc OFFSET 50 LIMIT 10
I found a previous conversation [1] suggesting to implement DISTINCT it as
a Hint, but that case could be solved by using the visitor (but I guess
they did not need sorting and paging).
Is not possible to do such query, am I doing something wrong or is it a bug
in the implementation?
Thanks in advance,
César
[1] http://osgeo-org.1560.x6.nabble.com/distinct-query-hint-td5049204.html
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César Martínez Izquierdo
GIS developer
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Blog: http://geotechnotes.wordpress.com/
ETC-SIA: http://sia.eionet.europa.eu/
Universitat Autònoma de Barcelona (SPAIN)
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