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https://issues.apache.org/jira/browse/JENA-803?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14259704#comment-14259704
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Andy Seaborne commented on JENA-803:
------------------------------------
An example:
{noformat}
// Functionality is defined by an accumulator,
// An accumulator is applied to all the elements in a group
static AccumulatorFactory f = new AccumulatorFactory() {
@Override
public Accumulator createAccumulator(AggCustom agg) {
return new Accumulator() {
int count = 0 ;
@Override
public void accumulate(Binding binding, FunctionEnv
functionEnv) {
ExprList exprList = agg.getExprList() ;
for(Expr expr: exprList) {
try {
NodeValue nv = expr.eval(binding, functionEnv) ;
if ( nv.isLiteral() )
count ++ ;
} catch (ExprEvalException ex) {}
}
}
@Override
public NodeValue getValue() {
return NodeValue.makeInteger(count) ;
}} ;
}
} ;
{noformat}
Custom aggregates must be registered by URI before use:
{noformat}
public static void main(String[] args) {
String aggUri = "http://example/countLiterals"; ;
Graph g = SSE.parseGraph("(graph (:s :p :o) (:s :p 1))") ;
AggregateRegistry.register(aggUri, f);
String qs = "SELECT (AGG <http://example/countLiterals>(?s,?o) AS
?X)\n{ ?s ?p ?o }" ;
Query q = QueryFactory.create(qs, Syntax.syntaxARQ) ;
try ( QueryExecution qexec = QueryExecutionFactory.create(q,
ModelFactory.createModelForGraph(g)) ) {
ResultSet rs = qexec.execSelect() ;
ResultSetFormatter.out(rs);
}
System.out.println(q) ;
}
{noformat}
Currently, parsing requires the {{AGG}} trigger. This is temporary until
further parser changes are made.
> Custom aggregates in SPARQL.
> ----------------------------
>
> Key: JENA-803
> URL: https://issues.apache.org/jira/browse/JENA-803
> Project: Apache Jena
> Issue Type: New Feature
> Components: ARQ
> Affects Versions: Jena 2.12.1
> Reporter: Andy Seaborne
> Assignee: Andy Seaborne
>
> Support SPARQL custom aggregates.
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