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| <h1 class="title">TDB Quad Filter</h1> |
| |
| <p>This page describes how to filter quads at the lowest level of TDB. |
| It can be used to hide certain quads (triples in named graphs) or |
| triples.</p> |
| <p>The code for the example on this page can be found in the |
| <a href="https://github.com/apache/jena/tree/main/jena-examples/src/main/java/tdb">TDB examples</a> |
| Filtering |
| quads should be used with care. The performance of the tuple filter |
| callback is critical.</p> |
| <p>See also |
| <a href="dynamic_datasets.html">Dynamic Datasets</a> to |
| select only certain specified named graphs for a query.</p> |
| <p>TDB will call a registered filter on every quad that it retrieves |
| from any of the indexes, both quads (for named graphs) and triples |
| (for the stored default graph). This filter indicates whether to |
| accept or reject the quad or triple. This happens during basic |
| graph pattern processing.</p> |
| <p>A rejected quad is simply not processed further in the basic graph |
| pattern and it is as if it is not in the dataset.</p> |
| <p>The filter has a signature of: <code>java.util.function.Predicate<Tuple<NodeId>></code> |
| with a type parameter of <code>Tuple<NodeId></code>. <code>NodeId</code> is the low level |
| internal identifier TDB uses for RDF terms. <code>Tuple</code> is a class for |
| an immutable tuples of values of the same type.</p> |
| <pre><code> /** Create a filter to exclude the graph http://example/g2 */ |
| private static Predicate<Tuple<NodeId>> createFilter(Dataset ds) |
| { |
| DatasetGraphTransaction dst = (DatasetGraphTransaction)(ds.asDatasetGraph()) ; |
| DatasetGraphTDB dsg = dst.getBaseDatasetGraph(); |
| |
| NodeTable nodeTable = dsg.getQuadTable().getNodeTupleTable().getNodeTable() ; |
| // Filtering operates at a very low level: |
| // need to know the internal identifier for the graph name. |
| final NodeId target = nodeTable.getNodeIdForNode(Node.createURI("http://example/g2")) ; |
| |
| // Filter for accept/reject as quad as being visible. |
| Predicate<Tuple<NodeId>> filter = item -> |
| { |
| // Quads are 4-tuples, triples are 3-tuples. |
| if ( item.size() == 4 && item.get(0).equals(target) ) |
| // reject |
| return false ; |
| // Accept |
| return true ; |
| } ; |
| return filter ; |
| } |
| </code></pre> |
| <p>To install a filter, put it in the context of a query execution |
| under the symbol <code>SystemTDB.symTupleFilter</code> then execute the query as normal.</p> |
| <pre><code> Dataset ds = ... ; |
| Predicate<Tuple<NodeId>> filter = createFilter(ds) ; |
| Query query = ... ; |
| try (QueryExecution qExec = QueryExecution.dataset(ds) |
| .query(query) |
| .set(SystemTDB.symTupleFilter, filter) |
| .build() ) { |
| ResultSet rs = qExec.execSelect() ; |
| ... |
| } |
| </code></pre> |
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