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| <h1 class="title">TDB Datasets</h1> |
| |
| <p>An |
| <a href="http://www.w3.org/TR/sparql11-query/#rdfDataset" title="http://www.w3.org/TR/rdf-sparql-query/#rdfDataset">RDF Dataset</a> |
| is a collection of one, unnamed, default graph and zero, or more |
| named graphs. In a SPARQL query, a query pattern is matched against |
| the default graph unless the GRAPH keyword is applied to a |
| pattern.</p> |
| <h2 id="dataset-storage">Dataset Storage</h2> |
| <p>One file location (directory) is used to store one RDF dataset. The |
| unnamed graph of the dataset is held as a single graph while all |
| the named graphs are held in a collection of quad indexes.</p> |
| <p>Every dataset is obtained via TDBFactory.createDataset(Location) |
| within a JVM is the same dataset. (If a model is obtained from via |
| TDBFactory.createModel(Location) there is a hidden, shared dataset |
| and the appropriate model is returned. The preferred style |
| is to create the dataset, then get a model.)</p> |
| <h2 id="dataset-query">Dataset Query</h2> |
| <p>There is full support for SPARQL query over named graphs in a |
| TDB-back dataset.</p> |
| <p>All the named graphs can be treated as a single graph which is the |
| union (RDF merge) of all the named graphs. This is given the |
| special graph name <a href="urn:x-arq:UnionGraph%5C">urn:x-arq:UnionGraph\</a> in a GRAPH pattern.</p> |
| <p>When querying the RDF merge of named graphs, the default graph in |
| the store is not included. This feature applies to queries only. It |
| does not affect the storage nor does it change loading.</p> |
| <p>Alternatively, if the symbol tdb:unionDefaultGraph (see |
| <a href="configuration.html">TDB Configuration</a>) is |
| set, the unnamed graph for the query is the union of all the named |
| graphs in the datasets. The stored default graph is ignored and is |
| not part of the data of the union graph although it is |
| accessible by the special name <code><urn:x-arq:DefaultGraph\></code> in a GRAPH |
| pattern.</p> |
| <p>Set globally:</p> |
| <pre><code>TDB.getContext().set(TDB.symUnionDefaultGraph, true) ; |
| </code></pre> |
| <p>or set on a per query basis:</p> |
| <pre><code>try(QueryExecution qExec = QueryExecution.dataset(dataset) |
| .query(query) |
| .set(TDB.symUnionDefaultGraph,true) |
| .build() ) { |
| .... |
| } |
| </code></pre> |
| <h2 id="special-graph-names">Special Graph Names</h2> |
| <table> |
| <thead> |
| <tr> |
| <th>URI</th> |
| <th>Meaning</th> |
| </tr> |
| </thead> |
| <tbody> |
| <tr> |
| <td><code>urn:x-arq:UnionGraph</code></td> |
| <td>The RDF merge of all the named graphs in the datasets of the query.</td> |
| </tr> |
| <tr> |
| <td><code>urn:x-arq:DefaultGraph</code></td> |
| <td>The default graph of the dataset, used when the default graph of the query is the union graph.</td> |
| </tr> |
| </tbody> |
| </table> |
| <p>Note that setting tdb:unionDefaultGraph does not affect the default |
| graph or default model obtained with dataset.getDefaultModel().</p> |
| <p>The RDF merge of all named graph can be accessed as the named graph |
| <code>urn:x-arq:UnionGraph</code> using |
| <code>Dataset.getNamedModel("urn:x-arq:UnionGraph")</code> .</p> |
| <h2 id="dataset-inferencing">Dataset Inferencing</h2> |
| <p>Inferencing on a Model in a Dataset, using the <a href="java_api.html">TDB Java API</a>, follows the same pattern as an in-memory InfModel. |
| The use of <a href="tdb_transactions.html">TDB Transactions</a> is <strong>strongly</strong> recommended to avoid data corruption.</p> |
| <pre><code> //Open TDB Dataset |
| String directory = ... |
| Dataset dataset = TDBFactory.createDataset(directory); |
| |
| //Retrieve Named Graph from Dataset, or use Default Graph. |
| String graphURI = "http://example.org/myGraph"; |
| Model model = dataset.getNamedModel(graphURI); |
| |
| //Create RDFS Inference Model, or use other Reasoner e.g. OWL. |
| InfModel infModel = ModelFactory.createRDFSModel(model); |
| |
| ... |
| //Perform operations on infModel. |
| ... |
| </code></pre> |
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