GH-5231: fix poor query performance for hasStatements() in FedX

The previous implementation of the FedXConnection was delegating
"hasStatements()" to the implementation of "getStatements()", where the
latter was actually fetching data from the federation members.

For checks hasStatements() checks like {null, rdf:type, null} or even
{null, null, null} the implementation is problematic as it would fetch
all data matching the pattern from the federation members, only to
answer if it actually exists.

We now make use of "existence" check on the federation members, and can
actually rely on the source selection cache for this.

Unit test coverage has been added.

Author: aschwarte10

tools/federation/src/main/java/org/eclipse/rdf4j/federated/FedXConnection.java

@@ -337,6 +337,27 @@ protected SailException convert(RuntimeException e) {
 		}
 	}
 
+	@Override
+	protected boolean hasStatementInternal(Resource subj, IRI pred, Value obj, boolean includeInferred,
+			Resource[] contexts) {
+		try {
+			Dataset dataset = new SimpleDataset();
+			FederationEvalStrategy strategy = federationContext.createStrategy(dataset);
+			QueryInfo queryInfo = new QueryInfo(subj, pred, obj, 0, includeInferred, federationContext, strategy,
+					dataset);
+			federationContext.getMonitoringService().monitorQuery(queryInfo);
+			return strategy.hasStatements(queryInfo, subj, pred, obj, contexts);
+
+		} catch (RuntimeException e) {
+			throw e;
+		} catch (Exception e) {
+			if (e instanceof InterruptedException) {
+				Thread.currentThread().interrupt();
+			}
+			throw new SailException(e);
+		}
+	}
+
 	@Override
 	protected void addStatementInternal(Resource subj, IRI pred, Value obj, Resource... contexts) throws SailException {
 		try {

tools/federation/src/main/java/org/eclipse/rdf4j/federated/evaluation/FederationEvalStrategy.java

@@ -562,7 +562,7 @@ public CloseableIteration<Statement> getStatements(QueryInfo queryInfo, Resource
 			IRI pred, Value obj, Resource... contexts)
 			throws RepositoryException, MalformedQueryException, QueryEvaluationException {
 
-		List<Endpoint> members = federationContext.getFederation().getMembers();
+		List<Endpoint> members = getAccessibleFederationMembers(queryInfo);
 
 		// a bound query: if at least one fed member provides results
 		// return the statement, otherwise empty result
@@ -605,6 +605,47 @@ public CloseableIteration<Statement> getStatements(QueryInfo queryInfo, Resource
 		return union;
 	}
 
+	/**
+	 * Returns true if the federation has statements
+	 *
+	 * @param queryInfo information about the query
+	 * @param subj      the subject or <code>null</code>
+	 * @param pred      the predicate or <code>null</code>
+	 * @param obj       the object or <code>null</code>
+	 * @param contexts  optional list of contexts
+	 * @return the statement iteration
+	 *
+	 * @throws RepositoryException
+	 * @throws MalformedQueryException
+	 * @throws QueryEvaluationException
+	 */
+	public boolean hasStatements(QueryInfo queryInfo, Resource subj,
+			IRI pred, Value obj, Resource... contexts)
+			throws RepositoryException, MalformedQueryException, QueryEvaluationException {
+
+		List<Endpoint> members = getAccessibleFederationMembers(queryInfo);
+
+		// form the union of results from relevant endpoints
+		List<StatementSource> sources = CacheUtils.checkCacheForStatementSourcesUpdateCache(cache, members, subj, pred,
+				obj, queryInfo, contexts);
+
+		if (sources.isEmpty()) {
+			return false;
+		}
+
+		return true;
+	}
+
+	/**
+	 * Returns the accessible federation members in the context of the query. By default this is all federation members.
+	 *
+	 * @param queryInfo
+	 * @return
+	 */
+	protected List<Endpoint> getAccessibleFederationMembers(QueryInfo queryInfo) {
+		return federationContext.getFederation().getMembers();
+	}
+
 	public CloseableIteration<BindingSet> evaluateService(FedXService service,
 			BindingSet bindings) throws QueryEvaluationException {
 

tools/federation/src/test/java/org/eclipse/rdf4j/federated/BasicTests.java

@@ -139,6 +139,61 @@ public void testBindClause() throws Exception {
 		execute("/tests/basic/query_bind.rq", "/tests/basic/query_bind.srx", false, true);
 	}
 
+	@Test
+	public void testRepositoryConnectionApi() throws Exception {
+
+		prepareTest(
+				Arrays.asList("/tests/basic/data_emptyStore.ttl", "/tests/basic/data_emptyStore.ttl"));
+
+		Repository repo1 = getRepository(1);
+		Repository repo2 = getRepository(2);
+
+		IRI bob = Values.iri("http://example.org/bob");
+		IRI alice = Values.iri("http://example.org/alice");
+		IRI graph1 = Values.iri("http://example.org/graph1");
+		IRI graph2 = Values.iri("http://example.org/graph2");
+
+		try (RepositoryConnection conn = repo1.getConnection()) {
+			conn.add(bob, RDF.TYPE, FOAF.PERSON, graph1);
+			conn.add(bob, FOAF.NAME, Values.literal("Bob"), graph1);
+		}
+
+		try (RepositoryConnection conn = repo2.getConnection()) {
+			conn.add(alice, RDF.TYPE, FOAF.PERSON, graph2);
+			conn.add(alice, FOAF.NAME, Values.literal("Alice"), graph2);
+		}
+
+		var fedxRepo = fedxRule.getRepository();
+
+		try (var conn = fedxRepo.getConnection()) {
+
+			// hasStatement which exist
+			Assertions.assertTrue(conn.hasStatement(bob, RDF.TYPE, FOAF.PERSON, false));
+			Assertions.assertTrue(conn.hasStatement(bob, RDF.TYPE, FOAF.PERSON, false, graph1));
+			Assertions.assertTrue(conn.hasStatement(null, RDF.TYPE, FOAF.PERSON, false));
+			Assertions.assertTrue(conn.hasStatement(null, RDF.TYPE, FOAF.PERSON, false, graph1));
+			Assertions.assertTrue(conn.hasStatement(null, RDF.TYPE, null, false));
+			Assertions.assertTrue(conn.hasStatement(null, RDF.TYPE, null, false, graph1));
+			Assertions.assertTrue(conn.hasStatement(null, RDF.TYPE, null, false, graph2));
+			Assertions.assertTrue(conn.hasStatement(null, null, null, false));
+			Assertions.assertTrue(conn.hasStatement(null, null, null, false, graph1));
+
+			// hasStatement which do not exist
+			Assertions.assertFalse(conn.hasStatement(bob, RDF.TYPE, FOAF.ORGANIZATION, false));
+			Assertions.assertFalse(conn.hasStatement(bob, RDF.TYPE, FOAF.PERSON, false, graph2));
+
+			// getStatements
+			Assertions.assertEquals(Set.of(bob, alice),
+					QueryResults.asModel(conn.getStatements(null, RDF.TYPE, FOAF.PERSON, false)).subjects());
+			Assertions.assertEquals(Set.of(bob),
+					QueryResults.asModel(conn.getStatements(null, RDF.TYPE, FOAF.PERSON, false, graph1)).subjects());
+			Assertions.assertEquals(Set.of(bob, alice),
+					QueryResults.asModel(conn.getStatements(null, null, null, false)).subjects());
+			Assertions.assertEquals(Set.of(bob),
+					QueryResults.asModel(conn.getStatements(null, null, null, false, graph1)).subjects());
+		}
+	}
+
 	@Test
 	public void testFederationSubSetQuery() throws Exception {
 		String ns1 = "http://namespace1.org/";