TDB Dynamic Datasets

TDB version 0.8.5 and later

This feature allows a query to be made on a subset of all the named graphs in the TDB storage datasets. The SPARQL GRAPH pattern allows access to either a specific named graph or to all the named graph in a dataset. This feature means that only specified named graphs are visible to the query.

SPARQL has the concept of a dataset description. In a query string, the clauses for FROM and FROM NAMED specify the dataset. The FROM clauses define the graphs that are merged to form the default graph, and the FROM NAMED clauses identify the graphs to be included as named graphs.

Normally, ARQ interprets these as coming from the web; the graphs are read using HTTP GET. TDB modifies this behaviour; instead of the universe of graphs being the web, the universe of graphs is the TDB data store. FROM and FROM NAMED describe a dataset with graphs drawn only from the TDB data store.

• Using one or more FROM clauses, causes the default graph of the dataset to be the union of those graphs.
• Using one or more FROM NAMED, with no FROM in a query, causes an empty graph to be used for the default graph.
• Using one or more FROM NAMED, with no FROM in a query, where the symbol TDB.symUnionDefaultGraph is also set, causes the default graph to be the set union of all the named graphs (FROM NAMED).

Example

#Follow a foaf:knows path across both Alice and Bobs FOAF data
#where the data is  in the datastore as named graphs.

BASE <http://example>
SELECT ?zName
FROM <alice-foaf>
FROM <bob-foaf>
{
   <http://example/Alice#me> foaf:knows ?y .
   ?y foaf:knows ?z .
   ?z foaf:name ?zName .
}